Feature request: allow merging and splitting of boxes

If the user selects more than one box, they could be given the option to merge them (using a hotkey, perhaps, such as `m`). And if they select one box, they could be given the option to split it (also with a hotkey, maybe `c` for "cut"?).

The merging would be particularly useful in situations like this one:

The text is almost perfectly identified, but the lines are super segmented. If, for whatever reason, a user absolutely needed one single box per line of text, they would currently have to delete all these boxes and draw a new one themselves. With the merge function, though, they could just select all the boxes and merge them! In certain situations, the merging would also automatically cover some words that weren't found in the first place, saving even more time.