dynamic parameter handling refactoring, part 2

This introduces a addDynamicFilter() method. We need to handle fixed and
dynamic filters separately for proper parentheises. The latter is not
implemented yet and will follow next.

Tests still broken

_test/mock/Search.php

@@ -14,6 +14,8 @@ class Search extends meta\Search
 
     public $filter = array();
 
+    public $dynamicFilter = array();
+
     /**
      * Register a dummy function that always returns false
      */