WIP: make `text_placement_info` not carry a reference to `text_placements` when all it needs are the defaults.
WIP: de-template `text_symbolizer_helper`.
Add `text_symbolizer_helper` constructor for `point_symbolizer`.
WIP: use `text_symbolizer_helper` for point symbolizer.
Extract "collidable" properties separately.

Previously these were bundled in with the text properties, which
duplicated several of them across different symbolizers and was an
obstacle to making `point_symbolizer` re-use common placement
code.

In bringing the "collidable" properties together and making them
common across uses of `placement_finder`, it brings support for
"ignore-placement" to TextSymbolizer and "avoid-edges",
"minimum-distance" and "minimum-padding" to PointSymbolizer. The
use of a common implementation should make it more consistent and
easier to modify and maintain.
Merge remote-tracking branch 'origin/mla-expr-v2-layout' into mla-expr-v2-layout

Conflicts:
	include/mapnik/renderer_common/process_point_symbolizer.hpp
	src/text/symbolizer_helpers.cpp