Support for wlr_tablet_tool

[heuer]

I still have support for tablets in the pipeline. wlr_tablet_tool is already implemented and required some changes especially to the Cursor class and ffi_build.

See main...heuer:pywlroots:tablet_feature

Considering the fact that WLR 0.17 is just around the corner: Should I open a PR for it now or wait for a WLR 0.17 release? I don't want to unnecessarily complicate the transition to 0.17.

@flacjacket, @jwijenbergh?

[jwijenbergh]

If you can, wait, as rebasing wlroots 0.16 changes on top of 0.17 is pretty difficult