Make more portable .vik file, as don't save the map cache directory if it's the map...

Make more portable .vik file, as don't save the map cache directory if it's the map cache default directory.

The map cache default directory is dependent on the user and OS, however on reading in when it's blank it automatically puts in the map cache default directory.

Unlike the previous effort SHA1: 39aec6c6b87a324981a6e1d028ba345320a444ad
This uses the fixed value MAPS_CACHE_DIR rather than the preference value in case the preference has changed.