- the name I was looking for for a long time :)
also less confusion with map layers
- move renderer.elements -> renderer.bucket
- rename RenderBuckets functions
- rename BucketRenderer.layers -> buckets
- no more duplication of TileLoaders for GWT ->
- decouple loadTile() from TileDataSource completed() call
- all TileDataSource MUST call completed(success) in any case now