Drop the subtypes, and keep a single udev-based GsdDeviceManager,
which will work on both backends, and should work on all platforms
we care about (?).
Now that we have tablet support in wayland, we can kind of honor
that in the places we're interested. The inverse (looking up the
GdkDevices that a single GsdDevice represents) is unneeded.
GsdWacomDevice has been updated, dragging GsdDeviceManager as a dependency
from g-s-d, which has been added to panels/common, and compiled as a
separate static libary, which is used by the wacom and mouse modules.
gsd-input-helper.[ch] is now in such library and has been removed from
the panel directories.
https://bugzilla.gnome.org/show_bug.cgi?id=743196
