Search in kf6/kmoretools where .desktop files are installed now

I noticed while running a KF6 Dolphin build that KMoreTools was finding my the system /usr/share/kf5/kmoretools/presets-kmoretools/fsearch.desktop instead of the one in the local kf6-qt6 KDE prefix. The latter has a path within the $prefix/share directory that starts with kf6, not kf5. The CMake variables are obstructing this fact a little by calling it ${KDE_INSTALL_DATADIR_KF} which isn't quite as clear, but hopefully that variable will stick to kf6 in the path for a long time. So, it should be okay to keep it hardcoded it in the .cpp files.