Allow reassigning shortcut from Define dialog

From https://musescore.org/en/node/249146#comment-777526

When assigning a new shortcut and it conflicts with an existing binding, allow unbinding the current assignment from the current dialog.
Having to cancel out of the Define dialog back to the shortcut list, possibly having to change the filter to locate the old assignment in order to clear it to then go back to where you were is simply not that user friendly.