Decrease selection

After #171 , in selection mode, if there is a selection and a new selection rect (green dashed one) is dragged over an already selected area while the key module META (Cmd or Ctrl) is pressed, then the area is removed from the selection.

It should work for several areas if new selection is dragged over several previously selected area.

Selection decrease should be done only when on `mouseup`, only when user is sure he want to alter the selection. If he drags the new selection rect away, the areas should be back to red (selected) and remain in selection list.