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!
methods are now in core
A new QgsVisibilityPresetCollection object has been created
which stores a set of visibility presets. QgsProject now
contains a preset collection object.
This allows plugins and other core objects to retrieve and
modify visibility presets.
