Allow mods to define a min/max engine versions for compatibility

In settings.txt, use lines like these (they're optional):

    version_min=0.19
    version_max=1.0
Don't install default mod in its own directory
Display more information in mod tooltips

Also, fix a bug with compareVersions()
Change game_prefix to save_prefix in misc.txt for clarity
Re-add .desktop file to distribution/