(ARCalibration): fix extrinsic calibration de-synchronisation

What does this MR do?

Ensure that the chessboard is detected in the two cameras before to add the calibration information. It prevents the de-synchronization of the calibration information.
Add an action in the extrinsinc view to save the calibration images.

How to test it?

• Launch ARCalibration/Calibration activity
• Perform the intrinsic calibration for the two cameras
• Click on the extrinsic button to open the extrinsic view
• Play the video and detect the chessboard
• Click on the Add button
• Check that the video are synchronized (you may put a smartphone with a chronometer on the chessboard)
  • You need to add multiple position, the bug didn't appear all the time

Some results

Before:

Now:

Associated Issues/Merge Requests

• Issues
  • Fixes #292 (closed)