Previously, we had two different approaches.  This seems like a simpler
approach.

We simply cache the Vimium CSS in chrome.storage.local (in the
background page) and fetch it from there (in UI components).

There is also a minor change in the we no longer cache the CSS in
memory.  This seems to be the right thing to do.  Caching the CSS in
memory consumes a small amount of memory.  However, it can only speed up
the fastest loads.  It cannot speed up the first load -- which is likely
the one that matters most.  So caching the CSS in memory seems to make
little sense.

Problems:
  - The meanings of some of the Mode/handlerStack constant names is far
    from obvious.
  - The same thing is named different things in different places.

This changes various constant names such that:
  - the names used in the handler stack and in the modes are the same.
  - ditto vis-a-vis DomUtils.

Also, break out the core of the handler stacks' `bubbleEvent` method
into a switch statements.  This makes it more obvious that the cases are
mutually exclusive.

This uses image data (instead of a path) for the page icon.  It also
only builds 19x19 and 38x38 icons, as per the
chrome.browserAction.setIcon() documentation.

This appears to fix a memory leak related to a recent Chrome regression
(versions 49+).

Fixes #2055.

Some of this code is showing its age, so these are just a number of
minor tweaks (to keep things clear, consistent and concise).

Also, add a couple of tests (while we're at it).

The intention is to use this to clean up some of the initialisation
sequences in the front end.