Add support for Steam + Humber Bundle / GOG Sources

[Starystars67]

The BeamMP Launcher needs to be able to support not just the Steam version but also versions of the game from other sources such as Humble Bundle and GOG.

Maybe something like the following process;
Attempt to launch via Steam URI
IF failed then
Check Registry for BeamNG.drive Location
IF failed then
Prompt user to select / provide the BeamNG.drive.exe file.