Use GsdDeviceManager to monitor libwacom-supported tablets coming and
going. Hide the Wacom panel from the list when there's no supported
tablets plugged in.
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