clutter/x11: Implement ClutterInputDeviceTool

This is implemented using Wacom-driver specific properties at
the moment, until libinput becomes the fallback driver handling
tablet and pad management.

Whenever a tool becomes in proximity, a new ClutterInputDeviceToolXI2
will be created (if it wasn't created previously) for the given
serial number. This tool will be set in all events send from the
device.

https://bugzilla.gnome.org/show_bug.cgi?id=773779