Fix image decoder on older OSes: Remove uses of 10.11 base64 APIs and fix sandbox permissions for 10.8.

Also uses the bitmap rep's dimensions instead of NSImage's dimensions, which are not always pixels (e.g., png can specify physical dimensions with the OS uses to scale the image down).

Only attempt "tmux recovery mode" (where a fake DCS code is sent internally to restart tmux integration on window restoration) if we successfully attach to a file descriptor server.

Block SIGPIPE in file descriptor server. It looks like macOS 10.12 broke syslog: when the parent process dies, syslog tries to write to a broken pipe.

The the tmux profile's background color but a little lighter/darker for the flexible view. Issue 4863.

row when there was a horizontal split. Issue 5233.

Fix a race condition with $ITERM_SESSION_ID where sometimes the pane number would be one too high, if the session got added to the tab before the pane number was requested.

Add PFMoveApplication, which offers to move the app to /Applications or ~/Applications on first launch

Nil out the delegate of the profile name field in the edit current session prefs panel because it gets made first responder from a delayed perform after its delegate is released (the delayed perform is in openToProfileWithGuid:selectGeneralTab:). When the text field tries to become first responder it checks with its delegate, and that dies when zombies are enabled.

Cap the number of commands, hosts, and directories saved per session to 100 to keep the saved state from getting too big. Issue 5029

Decode images out of process. This is for improved security because buffer overflows when decoding images are a real risk. This commit adds a new executable, image_decoder, which reads a compressed image on fd 0 and writes JSON describing its frames, sizes, and delays (for animated GIFs) on fd 1. When an image is received, the image_decoder job is run in a sandbox. Likewise, when iTerm2 restores saved state, image_decoder again decodes each image. In this way images that were not in our control are always passed through image_decoder. Some images, like the broken pipe graphic, are under our control and do not need to be decoded out of process. When restoring state, one image at a time is decoded in order of most recently requested. This provides adequate performance while minimizing latency.