Simplify cross-compilation of qtwaylandscanner_kde

I am trying to cross-compile and since qtwaylandscanner_kde is used during the build, I need to build a native version of the qtwaylandscanner_kde tool first. This change allows building qtwaylandscanner_kde as a standalone CMake project which the cross-build can then be pointed at to find the host tool. If an explicit path to qwaylandscanner_kde is not passed, we will try to use ExternalProject_Add to build it. This approach was taken from ksyntaxhighlighting: https://invent.kde.org/frameworks/syntax-highlighting/-/blob/master/src/indexer/CMakeLists.txt

This patch makes it significantly easier to cross-compile KWin, as we now only need a native QtCore library instead of all the dependencies.

This was previously submitted as kwayland-server!302 (closed)