Refine composer shortcuts

This fixes editing shortcuts like Ctrl+I breaking when clicking on toolbar buttons, a crasher because the composer left the undo button enabled after it was closed, and allows Escape to close the composer from anywhere.

Also fixes https://bugzilla.gnome.org/show_bug.cgi?id=774651, https://bugzilla.gnome.org/show_bug.cgi?id=785187, and https://bugzilla.gnome.org/show_bug.cgi?id=741741