Offer to merge label files on saving to existing file

From the GUI, saving labels to file overwrites the file if it already exists.

Such a situation does not usually happen because the default file name includes the current date and time, preventing name conflict.

However, to maintain some ground-truth behavioral data, it was previously suggested to save the result of manual tagging into a groundtruth.label file, for each experimental run/assay separately.

Such a file would have to be updated in the case of partial ground-truthing. Indeed, a tagger can be trained on quickly labelled data, tested on the same data, and the predictions be manually corrected afterwards in an attempt to retrain and improve the tagger.

From the GUI, to export such manual corrections, instead of generating a new .label file and calling the merge command for this file and the local groundtruth.label file, the user could input the name of the existing ground-truth .label file, and idealy this should result in the latter file to be updated with the new labels instead of being overwritten.

Proposal: On saving to an existing file, open a second pop-up window to ask the user whether to merge the new labels into the file or overwrite the file (two buttons).