Add configurable graphics sets

[Tommalla]

Add sets of graphics to the config file and allow the user to switch between them in Settings.
The general idea:
A set defines graphics as they are now in graphics section. There would be a field containing the name of the set currently in use.
Idea : Add additional configurability to a set eg.: allow for defining more complicated background, a set of different pawns, different balls etc.
Maybe even take it a little bit further and let the user select the set of pawns in use when he/she is beginning a new game?