WIP: fire autocomplete on key-press instead of key-release

Move TAB handling into connect_key_press_event . This also subjectively speed-up UI response.

At this moment, this drops support for @ . I haven't implemented it yet.

Note: also it's similar spaghetti code as before, but with my actual Rust knowledge I'm not able to improve it much.

Closes: https://gitlab.gnome.org/GNOME/fractal/-/issues/168

Signed-off-by: David Heidelberg david@ixit.cz