Accept dead keys as hotkeys. Issue 6102.

This fixes both deadkeys alone (e.g., ` on a DE keyboard) or deadkey+key (opt+` on DE to give '). I tested migration from 3.0 and it seems fine.

sources/iTermHotkeyPreferencesModel.m

@@ -56,7 +56,7 @@
 - (BOOL)hotKeyAssigned {
     BOOL hasAlternate = [self.alternateShortcuts anyWithBlock:^BOOL(iTermShortcut *shortcut) {
-        return shortcut.charactersIgnoringModifiers.length > 0;
+        return shortcut.charactersIgnoringModifiers.length > 0 || shortcut.characters.length > 0 || shortcut.keyCode != 0;
     }];
     return (_primaryShortcut.isAssigned ||
             self.hasModifierActivation ||

sources/iTermShortcut.m

@@ -187,11 +187,16 @@ const NSEventModifierFlags kHotKeyModifierMask = (NSCommandKeyMask |
 }
 - (NSString *)stringValue {
-    return self.charactersIgnoringModifiers.length > 0 ? [iTermKeyBindingMgr formatKeyCombination:self.identifier keyCode:self.keyCode] : @"";
+    // Dead keys can have characters without charactersIgnoringModifiers. For example, option+` on
+    // a German keyboard enters an apostrophe ('). The ` key is a dead key, so if you ignore modifiers
+    // it's nothing. So if there are either characters or characters ignoring modifiers, it's a
+    // formattable shortcut. If you press just the dead key then both characters and charactersIgnoringModifiers
+    // will be empty.
+    return (self.charactersIgnoringModifiers.length > 0 || self.characters.length > 0 || self.keyCode != 0) ? [iTermKeyBindingMgr formatKeyCombination:self.identifier keyCode:self.keyCode] : @"";
 }
 - (BOOL)isAssigned {
-    return self.charactersIgnoringModifiers.length > 0;
+    return self.charactersIgnoringModifiers.length > 0 || self.characters.length > 0 || self.keyCode != 0;
 }
 - (iTermHotKeyDescriptor *)descriptor {