popup-menu: Add 'sensitive' flag to item parameters

A menu action may not make sense at any time, so add API to mark
an item insensitive to indicate that its action is currently
unavailable, but may become activatable at a later point.

https://bugzilla.gnome.org/show_bug.cgi?id=659270

data/theme/gnome-shell.css

@@ -164,6 +164,10 @@ StTooltip StLabel {
     background-color: #4c4c4c;
 }
 
+.popup-menu-item:insensitive {
+    color: #9f9f9f;
+}
+
 .popup-image-menu-item {
 }