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!
* With a selection model, the way the attribute table handles selections
can be customized. E.g. synchronized to layer selection or used to pick
features.
* With request filters, the visible features on an attribute table can be
limited. This will effectively reduce the subset of features the attribute
table works on. Additional filters by means of a proxy model can of course
further reduce the visible subset subsequently.
