panelMenu: add a gap between the panel and its menus

The specs call for a 2 pixel gap between the panel and its menus,
though we need to specify this as 4 pixels, since it's relative to the
bottom of the icon/title, not the bottom of the panel (up until now,
the point of the menu arrow was actually overlapping the menu's
highlight underline).

Also, move the gap specification into the CSS, since it makes more
sense there.

https://bugzilla.gnome.org/show_bug.cgi?id=655627

js/ui/appDisplay.js

@@ -609,7 +609,7 @@ AppIconMenu.prototype = {
         if (St.Widget.get_default_direction() == St.TextDirection.RTL)
             side = St.Side.RIGHT;
 
-        PopupMenu.PopupMenu.prototype._init.call(this, source.actor, 0.5, side, 0);
+        PopupMenu.PopupMenu.prototype._init.call(this, source.actor, 0.5, side);
 
         // We want to keep the item hovered while the menu is up
         this.blockSourceEvents = true;