Better logic for determining Beat Saber installation directory

[VentyCZ]

This PR uses the following steps:

1. Finds out the Steam installation directory from registry (32/64bit)
2. Goes through all the defined library folders
3. Finds the library folder containing Beat Saber and uses it

[kinsi55]

Hey, thank you. I'll try to look into this in a bit

[VentyCZ]

@kinsi55 Thank you :) Haven't done any C# in a long while, so even any nitpicks on the code are appreciated!

[kinsi55]

Since you asked for it, I nitpicked :P

LGTM tho, I'll include it in the next update whenever that will be, thank you!

[VentyCZ]

Np, thanks for the nitpicking too :)

Mostly code style stuff, except for using LINQ - haven't done any C# in looong while, so I totally forgot how to LINQ ;)