Add ext-data-control protocol

This protocol allows a privileged client to control data devices. In
particular, the client will be able to manage the current selection and take
the role of a clipboard manager.

This is a straight port from wlr-data-control-unstable-v1 to ext-, as it
has not changed in five years and has near-universal compositor adoption.

Signed-off-by: Neal Gompa <neal@gompa.dev>