Add French "départements" as overlay layer

[davidzbiral]

For better orientation and because some settlement homonymy could be solved by seing the départements, adding départements - in the same way as cantons are already there - should be considered.
Optionally - but not if it should delay the solution - the issue of colors can be considered: it could be useful to have cantons as overlay in different color than départements, otherwise the département borders will get lost in the many canton borders.