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!
- New class QgsComposerObject, which both QgsComposerItem and QgsComposerMultiFrame
derive from. This class contains the framework for data defined composition properties.
- New class QgsComposerUtils, containing helpful static functions previously in QgsComposerItem
- Test suites for data defined settings in QgsComposerObject and functions in QgsComposerUtils
