virtual-pointer: introduce new protocol

This protocol is sponsored by wlroots for inclusion in the wp namespace. We consider this protocol on-topic for the wp namespace as it is a "plumbing" extension of the wl_pointer interface.

The following open-source implementations are available:

wlroots (server): https://github.com/swaywm/wlroots/blob/master/types/wlr_virtual_pointer_v1.c

The wlroots interface is currently supported by Sway.

wlroots (example client): https://github.com/swaywm/wlroots/blob/master/examples/virtual-pointer.c

wayvnc (client): https://github.com/any1/wayvnc

Acked-By: Drew DeVault sir@cmpwn.com