Make source code more relocatable

Occasionally, I see complaints about the file organization of kwin,
which is fair enough.

This change makes the source code more relocatable by removing relative
paths from includes.

CMAKE_CURRENT_SOURCE_DIR was added to the interface include directories
of kwin library. This means that as long as you link against kwin target,
the real location of the source code of the library doesn't matter.

With autotests, things are not as convenient as with kwin target. Some
tests use cpp files from kwin core. If we move all source code in a src/
directory, they will need to be adjusted, but mostly only in build
scripts.