zoom out

closes #177 (closed) #281 (closed)

• Shift+Click will zoom out for all windows by undoing a zoom in. Notice that in TPS, the task of zoom in/out is done by moving zoom markers to the corresponding position.
• Ruler marker and its text will stay at its corresponding position either with zoom in/out.
• When adding/removing a channel only the current window will be replotted with the last zoom range. (Please test on Q330 5244.sdr's VEP to see if the result is acceptable)