Add threading support.

This adds two new classes, class Thread which is taken from Qt's
movedobject tutorial and class OsmTileClusterRenderer, which is the
worker object class.

OsmTileClusterRenderer mostly consists of code moved from
NasaWorldWindToOpenStreetMapConverter.

NasaWorldWindToOpenStreetMapConverter's main responsibility now is
creating the threads and worker objects and assign new work to them.