Instead of having different marker sizes in legend as separate legend nodes,
the new "collapsed" mode packs all sizes into one legend node.
This commit only makes it available in the API, not exposed in GUI yet.
This is necessary in order to be able to correctly translate between absolute and relative paths
deeper in the code - e.g. paths to SVG files used in marker or fill symbols.
Until now, relative paths were translated to absolute paths on the fly.
This is now changed - paths to files should be always absolute within QGIS objects - and paths
only get turned into relative when saving projects. When loading a project, relative paths
are translated to absolute paths immediately.
This should lower the overall confusion about relative/absolute paths within QGIS, and also
allow having different base directories for relative paths (e.g. QML or QPT files may use relative paths
to their directory - rather than to the project directory)
The order of the elements is irrelevant and duplicate elements are unwanted. It
is therefore a perfect candidate for a set instead of a list. This prevents
filtering for duplicates manually be replacing some filer codes with (more
performant) builtin methods of QSet.
These were unused (since they were moved to data defined properties
at the symbol layer level) and were not set anywhere in
core QGIS (ie, the only way to set them was by manually calling
this api)
