Don't include system versions of wayland protocol

As long as we include the system versions of wayland (wayland-client.h and wayland-server.h instead of wayland-client-core.h and wayland-server-core.h) we can't upgrade wayland.xml to a version that's newer than the minimum required libwayland version for Qt Wayland (currently 1.6). That's a problem, because we want to implement newer protocol.

To fix this, we have to change all the includes to the core versions, and use wayland-scanner generated files where we actually need to use the protocol.

We also need to bump the minimum required libwayland version to 1.8.0, as that's when the -core includes were introduced.