UI sections: Support per-section pre-filter

In addition to the main sectionPreFilter in the cloudStack UI, allow a
per-section 'preFilter' which is called if the navigation section is
not returned in sectionPreFilter. This is used primarily to support
plugin sections which will not be displayed by default.

ui/scripts/ui/core.js

@@ -40,7 +40,9 @@
 
     $.each(args.sections, function(sectionID, args) {
       if (preFilter && $.inArray(sectionID, preFilter) == -1) {
-        return true;
+        if (!(args.preFilter && args.preFilter())) {
+          return true;
+        }
       }
 
       var $li = $('<li>')