Support virtual keyboard protocol

Created by: dcz-purism

This change is to support the recently proposed virtual-keyboard-v1 protocol.

Because virtual keyboards are privileged, the commits setting up an authorization system - meant to be general for all input methods - are separate.

The main commit provides support in wlroots as well as rootston. The keyboard does not attempt to synchronize its modifier status with the rest of the system, and code for having multiple instances running at the same time has not been extensively tested, however it's rather simple and wasn't failing either.