gog-extract-mac: add script to set up a mac app from gog

Grabs the app or apps from the dmg, unpacks the extras zip files, any extras
other than music/soundtracks get renamed somewhat sanely and shoved into the
Extras directory, either inside the app bundle if just one app, or inside
a subdir with the apps if more than one, and then any documents/text files/etc
in the game install directory get symlinked into the app bundle Extras dir.

Signed-off-by: Christopher Larson <kergoth@gmail.com>