Add a "game" variable to mod settings

This allows grouping mods together (e.g. fantasycore, alpha_demo, and
minicore are all part of flare-game).
Remove usage of the --game flag

Since games are determined via a mod setting, this flag is obsolete.
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/
Merge pull request #1115 from dorkster/gameflag_refactor

Improve mod metadata; Remove --game command line flag