Draft: Implement ext-virtual-keyboard-v1

This is derived from the implementation of virtual-keyboard-unstable-v1 and implements ext-virtual-keyboard-v1 as described here: wayland/wayland-protocols!211

I've added a repeat_info request to the protocol so that repeat events may be handled by the virtual keyboard provider rather than the client. This pretty much implements wayland/wayland!368.

Also implemented in wayvnc here: https://github.com/any1/wayvnc/tree/ext-virtual-keyboard-v1