Make 3rd-party-theme fallbacks work correctly, part 1:

For themes that don't inherit Oxygen, we either need to hardcode it as fallback, or revive the /usr/share/icons/default.kde symlink from KDE 3.

jstaniek indicated that there might be a workable solution also for Windows (using "shortcuts"), so I'm confident that the symlink is better. Unix only for now, and renamed to "default.kde4".

Thanks to dfaure for the advance work on the CMake script.