Add progress bar during import

[Gaudeval]

When importing an observation trace or PML classifications, progress is only notified to the user through the loading status on the import button. Progress should be more explicit with:

• a bar depicting progress over all imported files
• a bar depicting progress over the current file

This would probably require importers to take as an additional parameter a progress bar to update.
This would probably require importers to perform a first pass through a file to compute its length.