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)
Rationale:
- there was a lot of large objects passed by value, so potentially
there's a speed bump from this
- even for implicitly shared classes like QString/QList there's still
a (small) cost for copying the objects when there's no reason to
- it's the right thing to do!
