Install KWaylandServer headers

There are use cases for the headers to be used, e.g. when implementing wayland-specific workflows from an Effect. In order to be able to use these, we also need to expose libkwin to be imported as it carries the interfaces' symbols. Then one thing led to another so we needed to bump the required cmake version to 3.20.0.

Signed-off-by: Victoria Fischer victoria.fischer@mbition.io

Draft because I want to test it a bit more.