Functionality changes to mix_grades.py.

- Argument -m dumps also the model letter of each exam.
- Argument -r rounds scores to the given number of digits.
- Remove the "extra grades" feature (argument -x).
Towards showing the list of exams in the session.

- When the session is opened, a list of thumbnails with all the exams is
shown.
- Starting a session does not automatically start the capture.
Turn off the camera when going back to session mode.
Editing of previously graded exams is now enabled.

Pending:
- Updating the thumbnails list with the changes (capture and id).
- Removing already graded exams.
- Check that the capture of edited exams gets always saved.
- The captures are being erroneously saved without the annotations.
Fix error in edited capture saving.

They were saved without the bottom annotations if edited before
grading in the session.
The current student appears first in the student selection dialog.
Small fix in DialogStudentId.
Update the exam thumbnail after exam edition.
Exam removal and continue actions in edit from session mode.

In addition, improvements to exam updating in edit from session mode.
Clear selection in thumbnails view when beginning to grade.
Redesign the exam menu (just the search option) back to home action.

The search action is not yet implemented (the button is always
disabled).

The back action takes the user back to the session home screen.
Merge branch 'sessiondb' into development