backends/x11: Fetch the tablet serial prop on device add
For tablet device, the tool was created when the "Wacom Serial IDs" prop changed values. This property does not exist on the xf86-input-libinput driver but v1.5.0 of that driver has a different property for the serial.
The serial is constant (the driver creates one X device per serial), so we can fetch it after device creation and set it then. For earlier versions of the driver we assign the random serial 0xffffffaa - good enough to have at least a tool.
This fixes the crash in #3120 (closed) - clutter_event_motion_new() overrides event->device to the tool's device (if any). Without a tool motion events use the Virtual Core Pointer instead and our source device is never added to the stage's priv->pointer_devices.
When we generate an crossing event (which uses the source device) we fall afoul of an assert in clutter_stage_update_device() that expects our source device to be in priv->pointer_devices.
Fixes #3120 (closed)
See xf86-input-libinput!50 for the MR that adds the properties.