Multi-label support

[taxe10]

The following lines of code were added to support single-label assignment:

## remove the previously assigned label before assigning new one
 for current_key in current_labels_name.keys():
     for file in name_list:
         if file in current_labels_name[current_key]:
             current_labels_name[current_key].remove(file)
 ##

These lines should be removed for multi-label tagging. Additionally, it is worth noting that in a previous state of this application, when an image was assigned 2 different labels, the app used to create a duplicate of the image display in the front-end with the colors of these labels - this should be reviewed in the next weeks.