Add preference for menu button offset on main panel

Offset will behave as following

• When menu is on left or center in main panel,
  offset will be position of menu button from start in respective box
  e.g., 0 -> at the very start, 1 -> menu will be 2nd element in box

• When menu is on right on panel
  offset will be position of menu from end in rightBox
  e.g., 0 -> at the very end, 1 -> menu will be at second last position

Implements #164 (closed)