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!
- moved QgsVectorLayer::SimplifyHint enum to QgsVectorSimplifyMethod
- use QFlags instead of int for hints
- added few notes about simplification being added in 2.2
