QgsNumericSortTreeWidgetItem had a giant TODO saying "make it work".
This makes it work, and adds some other useful features like
being able to specify custom sort value and force items to always
sort on top.
- 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
