Add ext-data-control-v1 protocol

This is identical to the wlr protocol, and Smithay has implementations
for both.

New clients should use the `ext` protocol where present. Not sure how
widely used it is yet, but we probably should have both for now.

src/wayland/handlers/data_control/mod.rs

@@ -0,0 +1,4 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
+mod ext;
+mod wlr;