common: Fallback to GDK device type detection for Wacom touchscreens

libwacom support around touchscreens is unfortunately bumpy, and the
"TOUCH" value in the "Wacom Tool Type" property doesn't discern direct
from indirect touch devices, so ultimately rely on the GdkInputSource
to set the "system-integrated" flag expected on touchscreens.
common: Fix 2 possible initialization warnings in GsdDeviceMapper

If a plugin got to register, and query the output of a device before
the mapper is initialized, 2 warnings could happen when hitting NULL
pointers.