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.)

When distributing hint descriptors, do not post a frame's own hint
descriptors back to the frame itself.  It already has them.

With regard to the message-passing cost only, this represents a speedup
of approximately 3/2 for link-busy sites like reddit -- several tens of
milliseconds for me.  There are other costs too (such as processing the
hint descriptors) bu these are not affected.

The check that an element as an href (for certain hint modes) can be
done earlier, thereby avoid the need to pass that information between
frames.

We do not need to install separate event listeners for every callback.
Just install one listener and keep track of the callbacks ourself.

This is clearer, and also determines the order in which callbacks are
called.  (Although, we don't rely on that currently.)

This also adds a tests.