Some refac for new PySide2 bindings...
Some refac for new PySide2 bindings
- made DataSourceItem iterable
- added some tree print function for debug
- fixed minor bug on DataSourceContorller which prevented from adding products in more than one call per provider
- improved IDataProvider for future refac
Signed-off-by: Alexis Jeandet <alexis.jeandet@member.fsf.org>