Asynchronous loading of map previews in the map chooser dialog

Added async loading of map previews in the map chooses dialog based on the idea from Pull request 5.

The problem with it was that while the items *were* added asynchronously to the list, the actual map loading was done in the GUI thread since the map was first loaded and added to the cache in the Draw() override of the MapPreviewWidget.

I have added a method that forces loading the map preview into the cache and that can be called from another thread.