Change "is installed" detection to look for manifest, rather than a specific exe file

This ensures the launcher doesn't erroneously suggest a game is "not installed" simply because the specific exe file it uses to launch the game is not present - as occurred with the ECW modern client.

Instead, it should rely on the presence of a manifest file to indicate a folder that should contain a game installation. This ties in with #28 and #113 and the new application servicing stack as a whole.