Matches what this commit did for g-s-d: https://git.gnome.org/browse/gnome-settings-daemon/commit/plugins/media-keys?id=fbf3c5aa366ef7212f209e123d4aae315a1a2a8e https://bugzilla.gnome.org/show_bug.cgi?id=695409
It is irritating to show two different system popups at the same time, so let switcher popups like alt-tab cancel out the OSD. https://bugzilla.gnome.org/show_bug.cgi?id=613543
Implement a basic OSD popup that shows an icon and optionally a label and a fill level. It is based on the existing OSD implementation in gnome-settings-daemon, which it will replace. https://bugzilla.gnome.org/show_bug.cgi?id=613543