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
Instead of either hard-depending on Tracker being installed, or crashing
when the Tracker GSettings schema is not found, make the configuration
insensitive if the schema is not available.
https://bugzilla.gnome.org/show_bug.cgi?id=687490