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)
This change is being driven by:
- need for consistent terminology in the api. Currently there's
a confusing mix of border/outline used
- need for consistent terminology in the gui - we have a mix
of "border"/"outline"/"pen"
- bringing QGIS into line with the standard terminology used
in other graphic editing programs
