Use KDE_INSTALL_MIMEDIR instead of custom XDG_MIME_INSTALL_DIR
KDE_INSTALL_MIMEDIR defaults to share/mime/packages, same as the manually set path here. Also actually use SHARED_MIME_INFO_MINIMUM_VERSION in the find_package call for SharedMimeInfo.
KDE_INSTALL_MIMEDIR defaults to share/mime/packages, same as the manually set path here. Also actually use SHARED_MIME_INFO_MINIMUM_VERSION in the find_package call for SharedMimeInfo.