The order and way include macros were structured was chaotic, with no
real common thread between files. Try to tidy up the mess with some
common scheme, to make things look less messy.
This struct keeps the server side information for the wl_tablet_manager
global resource. It keeps the clients requesting this interface, and
does keep track of the plugged tablet devices, so
wl_tablet_manager.device_added is emitted on the expected clients.
