Implemented DragAndDropLayerListWindow being capable of drawing a reorderable layer list
Added new property to LayerMessages.java used by DragAndDropLayerListWindow
Implemented DragAndDropLayerList creating a reorderable ListGrid of raster layers which is capable of reordering the drawing order of the map
Implemented methods getRasterLayers() and getRasterLayer(String layerId) in MapModel making it possible to just get RasterLayers
Added missing properties to layer-gwt
Synchronize layer order, if order was changed from extrenal
Merge pull request #27 from lat-lon/dragAndDropLayertree-191

Drag and drop layertree 191