Move the vomnibar to an iframe

This PR

* moves the vomnibar to an iframe (to fix #599).
* merges #981 (to retain bookmarklet functionality, as well as to fix #968).
* tweaks #981 to fix an error in `hasChromePrefix`, where any string starting with `"about"`, `"view-source"`, `"chrome-extension"` or `"data"` would be passed to the tab opening functions without being converted to a search URL.
* introduces a few hacks to keep the vomnibar test functioning as before.
It might be helpful to add a comment in this regard, so we know in the future why it's here.