move car-calculation into separate thread
cleanup code
check if car left center-cluster on each gameplay-thread iteration

also fix and extend CoordinateTest
reload clusters if car left center-cluster

kinda wonky, but works
fetch new clusters in background thread

reloading clusters on demand now looks less wonky, but there's still
lots of work to do
automatically scroll viewport, wrap JXMapViewer in MapManager
Merge pull request #11 from TomTasche/feature/5

close #5: scroll viewport by one cluster-size