feat: Add key binding to toggle workspace panel

This fairly simple modification adds a key binding (^Y) to toggle display of the workspace panel, which would hopefully satisfy #11068. Also included is an update to the shortcuts window to include it. This was originally posted as a patch file in the Gajim MUC around the release of 1.4, but I hadn't gotten around to submitting it until now.

I'm not overly attached to the specific keybinding, but it wasn't in use. If anyone has compelling arguments for a different binding I'm all ears.