GRIM: Verify the game data at the first startup

This PR adds a check of the game data on startup, when the config option "check_gamedata" is true. It is true by default, so the data is verified at the first startup and then check_gamedata is set to false.
A dialog with a progress bar explains the user what's going on.

This still sets check_gamedata to false even if some data was corrupted or missing, as i feel it would be a little pushy to run the check every time until the user doesn't fix his data.