Tablet grabs
Created by: Ongy
Add grabs to the tablet-v2 input device structs.
Also use the tablet-tool grab to implement the logic to not change focus when a button is held or the tip is pressed down.
Currently buttons aren't tracked, only the tip works for now.
Testplan:
Use an application that gives feedback on tablet-tools (e.g. the example client in https://github.com/Ongy/wlroots/tree/tablet-v2-example) [which I should merge at some point] and start two instances. Hold pen over 1 window and connect tip. Move over other and notice that focus stays with the first.
Release pen over 2nd window and focus should be updated automatically (though no rotate/tilt/pressure/motion etc.)