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!
- call reserve on container classes where applicable
- make sure Q_FOREACH uses references
- remove dynamic casts to base classes
- fix some implicit bool conversions
