This change effectively reverts DWL to the prior development model, where development of DWL itself happens against the current wlroots release, and changes in anticipation of the next wlroots release are made on a separate branch.

Enables the protocol documented at
https://wayland.app/protocols/ext-data-control-v1

It is the upstreamed version of the old wlr_data_control.

It is used e.g. by mpv to read and write the clipboard.

User @thanatos has offered to take on the lead developer role for dwl.