remove art correctly

- memory stabilized because of art not being correctly removed.

MediatorState.destroy() is supposed to call refreshPoolObjectArt() in a way that all art gets removed. But only recycled art would be removed (because it uses poolobject.head) and because we disposeAll when we clear() then it wouldn't destroy pooled object and so would keep in memory all Arts...
Its not clear whether that was intentional to dispose of the graphics in another way or not.
But now memory doesn't stabilize and drops with time.

before, changing states quickly would result in going up to 400Mb ram used in 10-15 seconds (by keeping a key down) , after, it stays arbitrarily (may depend on machines) under about 200Mb when the garbage collector comes in and the graph shows clear action of the collector when previously it didn't.