fixed :open completions for slow computers. We now yield

incremental results, and the user gets more completion, the more often
he presses tab. Need to find out, how to change the "Waiting..." message
to "More results...". Also reminds me, how ugly the completion context
code is partly. We should try to clean it up and remove/merge useless
function to make it's interface smaller and more side-effect free.

common/content/liberator.js

@@ -1362,6 +1362,7 @@ const liberator = (function () //{{{
                 callback.call(self);
         },
 
+        // TODO: interruptable not used?
         threadYield: function (flush, interruptable)
         {
             let mainThread = services.get("threadManager").mainThread;