NOTE: `appstream-util verify` requires the XML file to have the correct
name in the source tree, so we can't simply have it as "metainfo.xml"
and rename on installation.
Other changes:
- rp-config.desktop: Rename on installation to:
com.gerbilsoft.rom-properties.rp-config.desktop
- DIR_INSTALL_MIME -> DIR_INSTALL_XDG_MIME; don't set it on Windows
or Linux.
- [xdg] CMakeLists.txt: Install based on the presence of the
DIR_INSTALL_XDG_* variables instead of the OS.