Adds a new visitor pattern API for creation of visitors which visit
all the style entities (symbols, color ramps, text formats, and
label styles) associated with different objects. Can be used on a
renderer, map layer, or project wide level.
E.g. on a project wide level, allows collection of ALL the style
symbols/color ramps/text settings inside a project, including those
in layouts or annotations!
For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
