Update for Gehn 20.
Add detection for unsupported OSes and warn the user.

This adds a dialog which warns the user if they're running the installer
under versions older than Windows Vista, which as of Gehn 20 is the
oldest supported Windows version.

It allows the user to continue installing in case:
- The installer is being run separately from the intended OS under which
  the game will be run.
- The detection fails.
Merge pull request #2 from GehnShard/XPDetection

Update to Gehn 20 and Detect Unsupported OSes