1ff31c0b6b
BackSpace does not work with `tools/entry.py` (made with GTK3), if the keymap with BackSpace does not contain any other keycodes. This should only happen for the emoji-layout or incomplete custom-layouts, because the layout-tests for normal layouts check for the presence of a button for Return. This does add an "Unknown"-keycode, if necessary, to let BackSpace work anyway. Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/694>