vector_layer and mvt_clone

[dbauszus-glx]

The vector_layer entry and mvt_clone entry type should essentially work the same but are quite different at the moment.

The vector_layer doesn't support themes.

There is a lot of duplicate code which is hard to maintain.

There should be a core layer entry into which the mvt_clone type uses to add to the clone specific items.

The clone logic isn't really required since an mvt layer can be created from the format method in a similar fashion to the vector layer.

It should be possible to create (draw) and modify geometries on vector and mvt [clone] layer.