Startup for beginners

[akks]

As we are introducing expert for to hide most complex features from beginners, we can also simplify first start screen of JOSM. Some of the people asks "Where is the Map???" and close JOSM (when they do not have enougn motivation to read something, for example scroll new features html to that description).

Maybe it would be good to open "Download from OSM" dialog at first startup (and add checkbox like "Run on startup" to it, turned on in non-expert mode by default). This feature need discussion of course.