Improve mod metadata; Remove --game command line flag

Closes #1111

- The directory structure in $HOME (e.g. `.config/flare`, `.local/share/flare`) to how it was in version 0.18. Local mods can be placed in `~/.local/share/flare/mods/`.
- In engine/misc.txt, `game_prefix` has been renamed to `save_prefix`
- In $MOD/settings.txt, there are some new keys. This information gets displayed in mod tooltips on the config screen.
    - **game** - This gets matched against the same key of the highest priority mod. So if alpha_demo is at the bottom of the mod list, only mods that have `game=flare-game` will be enabled.
    - **version_min** - This is the minimum engine version required to use this mod.
    - **version_max** - This is the maximum engine version required to use this mod.
- I've restored the .desktop file, since there's no more reason for games to provide their own.