ModManager improvments

Aside from some removal of duplicate code, this PR has two new features. Both can be added to mods via `mods/$MOD/settings.txt`

* **Mod descriptions**: This lets us provide some short details about what a mod does. It's displayed as a tooltip in the listboxes on the mod configuration screen
* **Mod dependencies**: This is the big one. It should help prevent cases where mod order breaks things (e.g. putting alpha_demo before fantasycore, or enabling minicore without both of those). See the description [here](https://github.com/dorkster/flare-engine/commit/a34c01d6d60781fe02210cbed1722d643fcd1892) for more details