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

js/ui/panelMenu.js

@@ -32,6 +32,15 @@ Button.prototype = {
     },
 
     _onButtonPress: function(actor, event) {
+        if (!this.menu.isOpen) {
+            // Setting the max-height won't do any good if the minimum height of the
+            // menu is higher then the screen; it's useful if part of the menu is
+            // scrollable so the minimum height is smaller than the natural height
+            let monitor = global.get_primary_monitor();
+            this.menu.actor.style = ('max-height: ' +
+                                     Math.round(monitor.height - Main.panel.actor.height) +
+                                     'px;');
+        }
         this.menu.toggle();
     },