Draft: Add input-observation protocol

Permits surfaces to observe input events also when not having input focus.

The draft includes support for monitoring pointer position; intended to extend for further input types and events (eg. touch positions). Input events are reported relative to an associated surface, and are intended to permit tools and gadgets such as xeyes with Wayland.

Justification

• This is an addition to the ext namespace. An (early) implementation is at https://github.com/phkaeser/wlmaker/tree/pointer-position.
• The implementation is under Apache 2.0 License. If it gets accepted, I intend to upstream into wlroots.

Requirements for merging

• Review
• Implementations
• ACKs from members