adds full tile load detection in MapTileDownloaded
adds TilesLoadedListener to MapView and field to store it
adds access from the MapDownloader to the tiles loaded listener in the mapview
added tile loaded listener example
moved listener subinterface and methods to the superclass
links downloader to listener method call, if listener is not null
alters constructors in MapTileProviderBase
alters constructors in MapTileProviderBasic