Allow copying from unified diff without the leading + and - chars

Copying chunks from the side-by-side DiffEditor works was expected. You can insert the lines anywhere directly.
For unified diff, however, you have the leading + or - chars, which are copied and inserted also.
It would be nice to omit them when copying, so it's easier to move code around (delete on one place, and pick it up from the diff to insert it on another place).