Changes .gitignore to ignore *.tmp
Fixes bug in table dialog, adds exclude attribute to graph model

- Fixes table dialog not applying the change of a comment
- On extraction excluded categories are marked as excluded in the graph
model
- Some code cleanup
Adds consideration of excluded attribute for JSON graph export
Adds exception handling

In most cases, the user is informed about failed save or load
operations.
Refactors error messages, fixes bug with loading graph