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.