Display number of picked particles in picking GUI

[Guillawme]

Hello,

It would be helpful if the picking GUI could display the current number of picked particles somewhere, to help the user know when they have enough picks to start training.

I tried wc -l on the CSV file with particle coordinates, but this file also has one line for each image without picked particles, so it's not straightforward to figure out how many particles were picked.

Thank you in advance. 😃