Install Instances in Custom Directories

[Koranir]

Fixes #373

[MCOfficer]

I'd much prefer if instead of doing it during the install, the option to change install dir is located in the settings. That's not in master yet, but there's a settings-tab branch you can work with.

[Koranir]

I was personally leaning towards the latter, but also requires us to move the install dir whenever the value is changed, which is a bit more effort in terms of UI.

It's easier to do the former (that's why I went that way in the first place), but if we really need the latter I'm willing to do it.

[MCOfficer]

remember this PR?

I butchered the git history locally 😕 github doesn't know it, but it is in fact merged.