This removes the direct dependency on GnomeRR while keeping the
existing functionality.
All the tiled monitor special casing was removed since the new API
doesn't expose tiled monitors in the first place.
A check for virtual size limits was removed but it wasn't being
enforced anyway.
We also introduce a blank error state for when the backend API isn't
available.
https://bugzilla.gnome.org/show_bug.cgi?id=782785
This makes loading faster, with less I/O, avoids unnecessary
code duplication (around 1k lines shaved), and ensures that
all the panels link and work appropriately.
By the same token, it will stop external panels from being
created, and loaded.
https://bugzilla.gnome.org/show_bug.cgi?id=690036