Most important changes:
- introduced feature iterator for QgsVectorLayer
- vector editing moved to QgsVectorEditBuffer
- complete rework of undo/redo commands for vector layers
- geometry cache separated from editing (QgsVectorLayerCache)
- non-essential editing functionality moved to QgsVectorLayerEditUtils
- update methods of existing classes
- add comment to methods missing in the sip bindings
- split up collective sip files into single files and use
same directory structure in python/ as in src/
- add a lot of missing classes (some might not make sense because of
missing python methods in those classes)
- remove some non-existing methods from the header files
- add scripts/sipdiff
- replace some usages of std::vector and std::set with QVector/QSet
