[composer] Refactor QgsComposerItem:
- 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