SQL initialization of MythGame settings variables

Certain MythGame plugin settings variables don't get initialized in the database. Then they can't be used unless they are first modified through MythGame settings UI. These variables are: GameAllTreeLevels, GameFavTreeLevels, mythgame.screenshotdir, mythgame.fanartdir and mythgame.boxartdir.
I've added these variables to the SQL code that populates the `settings` table.