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!
Previously, annotation size and position always used pixel units. This
did not work well when exporting layouts, resulting in tiny annotations
(it also caused issues when moving projects between hidpi/non hidpi
displays).
Instead, use millimeters for annotation size and position so that the
appearance is consistent across displays and works correctly in layout
exports.
Add lots of unit tests covering this.
Fixes#18373
