Move SearchEngines to bg-utils.coffee.

`SearchEngines` was previously in `utils.coffee`, which means it was
loaded in *every* content frame.  This is unnecessary, since it is only
used on the background page.  So this PR moves it there.

Also:

- Simplify some unnecessarily complex logic in `vomnibar.coffee`.
- Re-use `Utils.parseLines()` to parse the custom search engine configuation text.

This tweaks the jsaction detection, in particular excluding elements
where the "actionName" is "_".  I see a lot of these, and clicking them
doesn't do anything.

Also, added corresponding tests.

When the implementation of windowIsFocused() changed, the tests started
failing.

(It's not clear how I didn't spot this sooner.  I've run the tests
countless times - and they passed - since that change was made.

This replaces c01d7eea8675f9a7d84999777e8de72244d687b6.

All UI components require the document to be ready.  So, here the
constructor waits for DomUtils.documentReady().  The diff looks big, but
mainly it's a result of changes in indentation in the constructor and in
hide().

Also, hide() now uses @postMessage() to post a null message.  This
forces hide to use @iframePort.use(), which ensures that hide()s cannot
overtake activate()s.

This continues #2100.

On a slow loading page, as the page is loading, hit `o` repeatedly.
Eventually, Vimium hangs.

We seem to have had this problem for quite some time (e.g. it's in
1.54).

This fixes the problem by ensuring that the Vomnibar is initialized
before launching it.

Also fix the same issue for the help dialog.

Filtered hints: better typed text handling.

When the user is typing a link's text, any mistyped character exits link
hints mode.  This makes little sense.  In practice, this usually happens
because the user mis-typed something.

Here, we ignore typed characters which do not match any hints.

(Also, add a test for this.)