SCUMM: Add support for Steam versions of Indy 3, Indy 4, Loom and Dig

As the title says, this pull request adds support for the Steam versions of Indy 3, Indy 4, Loom and Dig.

I've based all the work on Ben Castricum's patch, but I've changed the way testapp.exe is handled so that it similar to how the disk images are handled for the NES, Apple 2 GS, C64 and Mac versions. Since this handling is done in the file handle level, the resulting file handling is less invasive than in Ben Castricum's patch, and the changes needed are small and self-contained.

I haven't added the CD audio handling code for Loom (in file CDDA.SOU). IMHO, these changes should be done at the engine level, rather than the audio CD manager level. If we are OK in modifying the audio CD manager level, these changes can be added as they are in a separate commit. Otherwise, the code for the handling of CDDA.SOU can be added in the SCUMM engine, rather than in common code (which is much cleaner, IMHO).

The original patch from Ben Castricum can be found at:
http://scummvm.bencastricum.nl/steam-lucasarts-support.patch
and an updated version for ScummVM 1.5.0 at:
http://gazj.co.uk/blog/2012/11/updating-the-steam-lucasarts-adventures-patch-to-work-on-scummvm-1-dot-5-0/
kPlatformSteam isn't a good idea -- Steam isn't the platform: It's still Windows/Macintosh. Ben even admitted in his original patch that it wasn't really a platform ;)