Add machinery for Nautilus to set up the trash state

Add machinery for Nautilus to set up the trash state

We add a method to say whether the trash is full/empty (to use the appropriate icon),
and a signal that the sidebar can use to request that the trash be emptied when
the user selects 'Empty trash' from the context menu.

Signed-off-by: Federico Mena Quintero <federico@gnome.org>