Installed GOG games detection

[Stew-rt]

I've noticed the installed game detection appears to work by looking for install directories by name.
It might be a better plan to look for the gameinfo files as they also appear to describe installed versions for future version detection.
These files appear to be consistent in gog installs.
Edit:
There's also the .mojosetup/manifests dir which has some potential for some future featuresets...

[tkashkin]

It's definitely a good idea, but it might add some performance hit.
It's probably worth to do more detailed checks, but not every time.
Maybe while initial games directory scan and while starting installation.
It also must be cached somehow.

Maybe additional scanning can be optional as looking for install directories by name will work totally fine if user does not install games manually.