create keyboard shortcuts for more commands in page segmentation window
i.e. combine selected lines (e.g. 'm' for 'merge') 'arrow-right' extends the currently chosen line on the right end in the same angle as the last segment by N pixels (user set value, default 5 or 10). 'arrow-left' shortens on the right end. 'shift-arrow-left' extends the currently chosen line on the left end. and 'shift-arror-right' shortens the currently chosen line on the left end. These values are given for RTL texts. LTR should be reversed. go to next-previous image (separate issue).