include would probably be more useful than exclude for `_importDXF`

Sketches are awesome. Importing DXFs is awesome.
`_importDXF` and its spawn seem kinda hard to use in practice. Generally, I know the names of the layers I want from a drawing. Some drawings have a lot of layers and generally I don't know the names of all the layers I don't want.
Since the function takes a list of layer names to exclude from the import, what I've been doing is figuring out the exhaustive list of layer names in the drawing (via my own calls to ezdxf), then subtracting away the names of layers I want. Doing that is alright, but I think it'd be more useful and intuitive if the function took a list of layers to retrieve rather than the list of layers to not retrieve.

https://github.com/CadQuery/cadquery/blob/6217431d9865ec4f0e0bb9ecf7dfd1aa204fe809/cadquery/occ_impl/importers/dxf.py#L160-L182