Invalid node labels and missing edge labels in saved Pajek data

- If node data is in a form of a Table, Save Network will use a repr of a row as a label, e. g. "[] {Sevnica}".
- Edge data is not saved.

Network save widget should have combos to choose the node and edge label when applicable.

Start by locating the Save Network widget and the Pajek serialization code, then reproduce the issue with Table-based node data and edge data. Done means the widget lets users choose applicable node and edge labels, saved node labels are not row reprs, and edge data is preserved.

Written by the indexing model from the issue text.