wayland/gtk-shell: Emit new capabilities event when changed

Merged Jonas Ådahl requested to merge jadahl/mutter:wip/gtk-shell-capability-changes into master

From the commit message of the last commit:

    The capabilities may change during the compositors lifetime, so make
    gtk-shell emit the capabilities event when capabilities change.
    
    https://gitlab.gnome.org/GNOME/mutter/issues/276

The rest are just some preparations and gobjectifications.

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