CMake: Explicitly check for required X11 libraries

CMake's FindX11 package reports that X11 was found even if not all libraries
are present. It also doesn't support proper components. This causes issues on
distros that split X11 headers into a myriad of small packages, for instance
Debian: some, but not all, components might be found.

Explicitly check that every X11 component used by Plasma was found by FindX11.