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This improves separation from the gnome-panel service scope.

This fixes the problem that the StatusNotifierHostRegistered signal was not
emitted after all Status Notifier Host applets were removed from the panel.

Depending on monitor resolutions panel position on screen can
change without changes in strut_size, strut_start and strut_end.

Check also geometry when deciding if strut has changed to make
sure that _GNOME_WM_STRUT_AREA is updated with correct values.

https://gitlab.gnome.org/GNOME/gnome-flashback/-/issues/95

And reorder code to make sure panel_button is created before
calling locations_changed.