remember 'current label', allow repeating it by default

remember the last label you entered; display this in the dialogue box and say '... or ...' e.g. if you don't enter a new label, it will use the last one; so it is fewer clicks / keyboard entries to repeat the same label many times (e.g. label all the trees, label all the pedestrians, label all the cars in sequence)

No file or test is named in the issue. Start by locating the label-entry dialogue and the code that handles submitted labels; confirm how the last label is stored and displayed. Done means an empty new-label entry reuses the displayed previous label, while entering a new label updates it.

Written by the indexing model from the issue text.