Add entry under Help to show open source licenses

Interfaces/MainMenu.xib

 <?xml version="1.0" encoding="UTF-8"?>
-<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="12121" systemVersion="16E195" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none">
+<document type="com.apple.InterfaceBuilder3.Cocoa.XIB" version="3.0" toolsVersion="13196" targetRuntime="MacOSX.Cocoa" propertyAccessControl="none">
     <dependencies>
-        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="12121"/>
+        <deployment identifier="macosx"/>
+        <plugIn identifier="com.apple.InterfaceBuilder.CocoaPlugin" version="13196"/>
     </dependencies>
     <objects>
         <customObject id="-2" userLabel="File's Owner" customClass="NSApplication">
@@ -1099,6 +1100,12 @@ DQ
                                     <action selector="copyModeShortcuts:" target="-1" id="fOs-EL-nnE"/>
                                 </connections>
                             </menuItem>
+                            <menuItem title="Open Source Licenses" identifier="Copy Mode Shortcuts" id="dV2-dF-UB1">
+                                <modifierMask key="keyEquivalentModifierMask"/>
+                                <connections>
+                                    <action selector="openSourceLicenses:" target="201" id="kij-nS-KMI"/>
+                                </connections>
+                            </menuItem>
                         </items>
                     </menu>
                 </menuItem>

iTerm2.xcodeproj/project.pbxproj

@@ -1920,6 +1920,7 @@
 		A6BDB0BC1B470CBE00F511E6 /* iTermRootTerminalView.m in Sources */ = {isa = PBXBuildFile; fileRef = A6BDB0BA1B470CBE00F511E6 /* iTermRootTerminalView.m */; };
 		A6C1207A1E39C3A4004021BB /* iTermBuriedSessions.h in Headers */ = {isa = PBXBuildFile; fileRef = A6C120781E39C3A4004021BB /* iTermBuriedSessions.h */; };
 		A6C1207B1E39C3A4004021BB /* iTermBuriedSessions.m in Sources */ = {isa = PBXBuildFile; fileRef = A6C120791E39C3A4004021BB /* iTermBuriedSessions.m */; };
+		A6C1FD4E1FC2A65E006B9A69 /* Licenses.txt in Resources */ = {isa = PBXBuildFile; fileRef = A6C1FD4D1FC2A65D006B9A69 /* Licenses.txt */; };
 		A6C4E8E21846E13800CFAA77 /* IntervalTree.h in Headers */ = {isa = PBXBuildFile; fileRef = A6C4E8DD1846E13800CFAA77 /* IntervalTree.h */; };
 		A6C537BE1938374600A08C18 /* iTermTabBarControlView.h in Headers */ = {isa = PBXBuildFile; fileRef = A6C537BC1938374600A08C18 /* iTermTabBarControlView.h */; };
 		A6C537C31939507100A08C18 /* iTermRestorableSession.h in Headers */ = {isa = PBXBuildFile; fileRef = A6C537C11939507100A08C18 /* iTermRestorableSession.h */; };
@@ -3756,6 +3757,7 @@
 		A6BDB0BA1B470CBE00F511E6 /* iTermRootTerminalView.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = iTermRootTerminalView.m; sourceTree = "<group>"; };
 		A6C120781E39C3A4004021BB /* iTermBuriedSessions.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = iTermBuriedSessions.h; sourceTree = "<group>"; };
 		A6C120791E39C3A4004021BB /* iTermBuriedSessions.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = iTermBuriedSessions.m; sourceTree = "<group>"; };
+		A6C1FD4D1FC2A65D006B9A69 /* Licenses.txt */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text; path = Licenses.txt; sourceTree = "<group>"; };
 		A6C4E8DC1846E13800CFAA77 /* IntervalTree.m */ = {isa = PBXFileReference; fileEncoding = 4; indentWidth = 4; lastKnownFileType = sourcecode.c.objc; path = IntervalTree.m; sourceTree = "<group>"; tabWidth = 4; };
 		A6C4E8DD1846E13800CFAA77 /* IntervalTree.h */ = {isa = PBXFileReference; fileEncoding = 4; indentWidth = 4; lastKnownFileType = sourcecode.c.h; path = IntervalTree.h; sourceTree = "<group>"; tabWidth = 4; };
 		A6C537BC1938374600A08C18 /* iTermTabBarControlView.h */ = {isa = PBXFileReference; fileEncoding = 4; indentWidth = 4; lastKnownFileType = sourcecode.c.h; path = iTermTabBarControlView.h; sourceTree = "<group>"; tabWidth = 4; };
@@ -4602,6 +4604,7 @@
 		1D03D42519144B8E0049EB8F /* ThirdParty */ = {
 			isa = PBXGroup;
 			children = (
+				A6C1FD4D1FC2A65D006B9A69 /* Licenses.txt */,
 				A65871661D85E6660061CEEE /* LetsMove */,
 				1D1F8C171A32616A00167161 /* AMIndeterminateProgressIndicator */,
 				F6441F610E748404000EC682 /* CGSInternal */,
@@ -7982,6 +7985,7 @@
 				A67D0D6B1A2EE12A003A8B35 /* iTermPasteSpecialWindow.xib in Resources */,
 				A68BADFC1E0DD52F00E4BB7B /* Touch Bar Function Keys@2x.png in Resources */,
 				1D4BAFEE1BFE77E4004FF52B /* iTermPrintAccessoryViewController.xib in Resources */,
+				A6C1FD4E1FC2A65E006B9A69 /* Licenses.txt in Resources */,
 				A6339DDC1D232DF2000BAD78 /* StatusItemAlt@2x.png in Resources */,
 				A67F118018D82B9500B23C7B /* PrefsAdvanced.png in Resources */,
 				9D737D6017495B4100B3334D /* PrefsGeneral@2x.png in Resources */,

sources/iTermApplicationDelegate.m

@@ -2045,6 +2045,11 @@ static const NSTimeInterval kOneMonth = 30 * 24 * 60 * 60;
     [[TmuxDashboardController sharedInstance] showWindow:nil];
 }
+- (IBAction)openSourceLicenses:(id)sender {
+    NSURL *url = [[NSBundle mainBundle] URLForResource:@"Licenses" withExtension:@"txt"];
+    [[NSWorkspace sharedWorkspace] openURL:url];
+}
+
 #pragma mark - Private
 - (void)updateProcessType {