Add polygon label placement options

[beroso]

Saw this PR about centered labels for polygons today.

I think it can be useful to add an option to place the label using Dart polylabel. It works well on both convex and concave polygons.

Example:

Centroid (current approach)	Polylabel

[JaffaKetchup]

Thanks for your PR! I'll have a proper look tomorrow :)
Just for now though, I can see that the enumerable names might need changing though, as they are a bit confusing. One question, might this have any impact on performance as far as you could see?

[JaffaKetchup]

This is working great! I've commented below on something(s) that might need changing, but I'm open to suggestions :)

[ibrierley]

I'm fine with this either way. It tests fine. It feels like each option in certain cases is preferable when I look at it visually, so no preference on which is default.

polylabel is maybe ambiguous as an parameter, but I guess it does what it says on the tin (uses whatever polylabel calculates, which could change and not be a center...). So no strong preference either way.

[JaffaKetchup]

LGTM