option to guess base path: use parent of map directory as base path

[illwieckz]

I made this option to make easy to test multiple games. Basically I extracted all pk3 from each game I have as one unpacked directory per game. So, opening some/path/to/maps/castle.map will use some/path/to as base path/

Note: I don't know how to disable the basepath option when this one is enabled.