Add wlr-data-control-unstable-v1

This is just a copy of wl_data_device_manager and its related interfaces, with drag-and-drop related stuff stripped out. The main difference is that the clipboard manager doesn't need to have keyboard focus to receive selection events.

I wonder if we need more than that in this protocol.

I intentionally left out primary selection for now, because it's easy to add but I want to make sure this design is good before doing so. Also some compositors don't support primary selection.

Fixes #20 (closed)