GRIM: Check the md5 of the game data files

This pull request adds a list of md5 sums for the game data files. Then the files loaded can be checked against them by calling ResourceLoader::checkFiles().
Currently there are the md5 of the demo and of the english and italian 1.0 retail. We'll need to ask on the forum for the other ones.
Also for EMI, but i think it's less urgent.

To check the files i have added a debugger command: 'check_gamedata'. Currently it does not accept any argument, so it will always check all the files.
If a file does not match one of its md5s a dialog will come up saying it's probably corrupt and with the md5 sum in case it isn't.