Update PrintPoints: content, export/import, iterators

[ioannaMitropoulou]

Each PrintPoint should know in addition to the current stuff also the following information:

• Index of layer it is coming from
• Index of path it is coming from
• Type of path it is coming from

The json export should be updated accordingly to contain this information. Instead of a flat dictionary, create a nested dictionary where each path and each layer has its own list.

Also the serialization (import + export) of the print organizer needs to be updated accordingly

Also, remove the current printpoints iterators, and add a new method (if needed?...) that uses the new data.