Use UIComponent for Vomnibar iframe
Fix tests for UIComponent Vomnibar iframe
Minor changes to vomnibar-in-iframe.

From top to bottom on the diff:

- The echo handler on the background page is no longer required.
- Simplify/refactor vomnibarUI message handler.
- Initialise vomnibar query to "" (rather than null) and simplify.
- No need to focus parent window when vomnibar closes; that's handled by
  the iframe framework.  Also no need to blur.