Support placement-type=list for Textsymbolizer

[gravitystorm]

https://github.com/mapnik/mapnik/wiki/TextSymbolizer

From mapnik 2.1, the placement types were expanded to dummy, simple and list. Only dummy and simple values are currently in the reference.

Also, the corresponding element also needs an entry in the reference somewhere, but it's not clear to me where that would go.

[springmeyer]

note the placement-type parameter also applies to the shield symbolizer and was added in ec739c8. But, the list type is still held back because we do not currently have a good way to represent this functionality in css. Ideas are welcome. If we have a solid CSS syntax then figuring out how to do this in mapnik-reference will come after.

[springmeyer]

see discussion at #49, which is relevant due to shared XML nesting of raster colorizer and alt placements.

[springmeyer]

Next actions (after merges above):

• - Docs for list type
• - Add list type for shields as well, for relevant mapnik versions
• - Still need to figure out a non-hacky way to expose in CartoCSS.

[gravitystorm]

For the docs, are you happy for the docs to explain the xml hackery?

For the non-hacky approach, that'll need some good head scratching. For example, when we have mapnik/mapnik#2138 to content with, we need to be able to say, for a given zoom please override the font size of a <Format> in a <Placement> in a <Layout>. I've no idea yet how to approach that.