utils/types: toggle_set: add "~~" semantics

Add support for "~~" special value that indicates that, when merging
into another set, all values from that set should in fact be dropped.
Apart from the unique merge semantics, "~~" just like any other "~"
value.

wa/utils/types.py

@@ -370,6 +370,9 @@ class toggle_set(set):
 
     @staticmethod
     def merge(source, dest):
+        if '~~' in dest:
+            dest.remove('~~')
+            return dest
         for item in source:
             if item not in dest:
                 #Disable previously enabled item