Ability to apply multi-seating

[mecattaf]

Related to #230:

Sway relies on libseatd, making it the only compositor I am aware of that natively supports multi-seating - that is the ability to assign a group of inputs (mouse/keyboard) to a specific seat.
I would like miracle-wm-input to be as powerful as sway-input otherwise it would be a big regression.
This includes setting custom xsettings per cursor.

[mattkae]

I believe that Mir might have support for multiple seats, but I'd have to research a bit further into what this would mean here and how it would get exposed