Carto updates for new text placement

[rundel]

@springmeyer, @herm

Not sure either of you have spent any time thinking about what the carto syntax will look like for implementing the new placement changes, but it is something that will need to be updated in the generator in the not too distant future.

Mapnik trunk currently produces a bunch of warnings due to the deprecated text symbolizer functions.

[springmeyer]

@rundel - yes, this definitely needs thought. Hopefully @herm is amenable to keeping the deprecated text_symbolizer functions (that should also work for the common use case) for the time being in the c++ api.