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
