panelMenu: Consider vertical margins when computing max-height

panelMenu: Consider vertical margins when computing max-height

When opening a panel menu, we set it's max-height to the available
work-area height to keep menus with scrollable content from growing
outside the monitor. However a menu that extends all the way down
to the bottom edge does not look great either, so also take margins
into account here.

https://bugzilla.gnome.org/show_bug.cgi?id=744498