Improved game-detection routines

[apocalyptech]

Not sure if this one's worth the effort, but it'd be nice to expand our game detection to be able to detect all installed instances of the game, regardless of store (or, at least, both Steam + EGS, if the user happens to have both). And then we'd be able to provide better button shortcuts in file dialogs, etc, in case folks want to manage a patch file on multiple installs. I'd also want the ability to actually save custom install locations, in the event that the autodetection doesn't work.

But basically just supporting an arbitrary number of installs per game, and Doing The Right Thing in situations where it'd be helpful.

This is obviously a little less important now that PythonSDK is the preferred method of mod installation, but pointing the user to the correct Binaries dir would be helpful regardless.