ComboBox and pop-up menus

[dhardy]

The ComboBox pop-up menus draw labels in the apparent margin. This is probably something to do with the hacky nature of pop-up menus (which use special layout logic in SubMenu).

• Option 1: use the same logic in SubMenu and ComboBox (the latter is essentially just the former with a different button face and selection logic anyway).
• Option 2: revise how menu layout works. It is the way it is (margins part of the entry) mostly because this seemed the easiest way to ensure sub-menus opened next to the parent menu (not overlapping the parent menu's margin).