Removed imageRepresentationTypeKeyPath,  imageRepresentationKeyPath, imageUIDKeyPath and imageTitleKeyPath from MMFlowView

- when providing content via bindings, the content items must conform to MMFlowViewItem protocol
- cleaner and simpler API
- prepared for contentAdapter infrastructure in MMFlowView which encapsulates bindings or datasource driven content access
- new protocol MMFlowViewContentAdapter
- new class MMFlowViewDatasourceContentAdapter implementing datasource wrapping
- refactored content binding to new class MMFlowViewContentBinder