Add Engine -> Browse for Engine

It would be awesome if we could add our custom UE4 engines to the launcher (NV-UE4 branch, Full Release Branch, Master Branch Build, etc…). This would also pave the way to an option for install locations for marketplace items and launcher engines.

+1 from men and my team, as we are partially using custom builds when developing UE4 Plugins for some of our internal projects + a couple intended for commercialisation at some point.