libgudev allocs a new GUdevDevice object for each event, so the pointer value
for the 'add' udev event differs from the one for the 'remove' event. If we
use the pointer value as hash table key, we'll never remove the device.
Switch to use the syspath of the device instead, that one is unique per
device.
Fixes#309
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
