Add possibility to choose which fields to export when saving to Excel. Also some testing and maintenance.

This pull requests includes the main feature (commit and pull request title are the same). It includes some refactoring (dialogs and images) and cleanup (removing autogenerated files)

Finally, it includes some testing as well:

In order to test:
```
make test
```

To test and see coverage reports (on console). Needs coverage (pip install -U coverage):
```
make coverage
```

To create html coverage reports:
```
make coverage-html
```