[Patch] Add MainPanel

[michael2402]

This is one more patch that restructures the Main class and pulls logic out of it.

A new class MainPanel is added to manage the MapFrame state and display the start panel. addLayer and removeLayer are now completely handled by the layer manager. I left the two special addLayer(layer, bounds) methods in main since they do not yet have a good replacement. The layer methods in main have been deprecated since they only proxy all calls to the layer manager now.

The layer manager got a new resetState() method. It can currently not handle failing removeLayer listeners, this is why the tests might still be in an unclean state. I'll work on that next week.

I tested this patch with several plugins but since the map view is now constructed after the layer was added to the layer list there might be problems with that. The MainPanel listener is registered before any plugin listeners so from their point there should be no difference. But you never know.