popupMenu: make submenus scrollable if needed

popupMenu: make submenus scrollable if needed

Right now, the network menu will overflow the screen if More...
is selected with many access points. As a short-term workaround
for this, add a scrollbar for submenus of panel dropdown menus
if they would cause the toplevel menu to overflow the screen.

- Put the actors in a PopupSubMenu in a StScrollView so we get
  a scrollbar if the allocated space is smaller than the height
  of the menu. Expand animation is turned off in the scrolled case
  to avoid weirdness.
- When we pop up a panel menu, set a max-height style property
  on the panel menu to limit it to the height of the screen.
- Hack event handling while the scrollbar is dragged to make
  the scrollbar work properly.

https://bugzilla.gnome.org/show_bug.cgi?id=646001