Only the management of tablets and tools is added so far. No tablet events
are yet interpreted.

As it's been the tradition in GTK+, erasers are split into their own device,
whereas the rest of the tools are meant to be routed through the
GDK_SOURCE_PEN device. Both pen/eraser devices are slaves to a master
pointer device, separate to wl_pointer's. This is so each tablet can
maintain its own cursor/positioning accounting.

Signed-off-by: Stephen Chandler Paul <thatslyude@gmail.com>