Replace libkeybinder by a GAction under Wayland

See [FeatHub vote here](https://feathub.com/Guake/guake/+29).

Under wayland, libkeybinder requires to force use of XWayland (X11 backend). This does not seem to do the job since some complain Guake does not show up if the current focussed application if a GTK3 application.

It seems the cleaner solution would be to develop a [GAction](https://developer.gnome.org/gio/stable/GAction.html) (see #1109) to replace the use of libkeybinder under Gnome/Wayland. This would not completely replace libkeybinder, since other DE (such as XFCE or KDE) won't see the "GAction". Maybe a plugin system should be developed to easily extend the global hotkey support in the future (`plugins/hotkeys/{gnome.py,kde.py,xfce.py,libkeybinder.py...}`).