grilo: Fix shutting down plugins

We were only unregistering sources, which isn't enough for
the plugins to get unloaded, and all their resources removed.
This was causing plugin cache directories not to be removed.
main: Shut down plugins on exit()

Due to plugins holding ref's on the engine, simply unref'ing the
plugin engine won't be enough for the plugins to shutdown.
rotation: Fix warning on exit

Due to the use of the wrong menu ID.
grilo: Fix warning on exit

Make sure the filter model is unref'ed before the model it
filters is. It doesn't like filtering models without a root.
data: Fix clipping of volume and go buttons