keyboard: Add Left Ctrl to Compose Key options
Adds an option to set Left Ctrl as the Compose key via the Keyboard panel. Resolves #1860 (closed).
Note that I targeted this MR to gnome-42, hoping that this could go out in a minor release, but I can rebase onto main if this isn't appropriate.
(Build appears to be failing because I made my fork Internal rather than Public. Will resolve that shortly.)