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!
Just like the earlier change which allows for text formats to
be managed in style manager, this change allows for layer-wide
label settings to be stored and managed in styles.
So while a text format includes just the font settings and other
appearance related settings, Label Settings also includes layer-type
specific settings such as label placement, priority, rendering
settings.
- also adds QgsStyleProxyModel which handles filtering of entities
- lots of unit tests
- new signals in QgsStyle for when symbols/tags/etc change (with tests)
