XdgDesktopFile: Reworks XdgDesktopFileCache::getFile()
XdgDesktopFile: Reworks XdgDesktopFileCache::getFile()

It adds null pointer protection and reuses code.
Returns nullptr if a file isn't found or isn't valid.

lxde/libqtxdg,lxde/libqtxdg,libqtxdg/libqtxdg,libqtxdg/libqtxdg,luis-pereira/libqtxdg,luis-pereira/libqtxdg