Rename some things to be consistent about how we treat highlight
states.

- Use blue highlights for focus no matter whether active highlights
  are on or off.
- Use `setFocused` instead of `setActive` when setting the focus.
- Rather than `setViewerFocus` and `setFocusedHighlights` have
  symmetrical RPC calls named 'focusAnnotations` in both directions.
- Get rid of the multiple highlight levels for focus. When showing the
  focus it's not important to darken inner highlights. The highlights
  visible mode still has multiple levels, all yellow.
- Remove the "annotator-hl-temporary" styles since we never call
  `setTemporary` anymore.
- Remove the "finalizeHighlights" event because nothing listens for it.
- Rename the scope method `isFocused` to `hasFocus` because I have a
  general preference against participial adjectives. It is much easier
  to remember always to use the basic verb or noun meaning of focus,
  with no ending. `hasFocus`, `focusAnnotations`, etc.

Removing dead code from our annotator branch and dropping the '-ed'
suffix from class names is left out for simplicity of managing a single
PR against one repo for the moment, but may be advisable.

So that
 - Instead of a map, we now store a list of focused tags
 - The check for being focused has been moved
   into a method in the controller
 - The sidebar now exepts simply a list of tags to
   focus on (no more add/remove tags to existing list)

Don't bother looking up the annotations (for the tags),
since we only need the list of tags anyway.

(It was different when we worked with the IDs, but we
moved to use tags instead, so the actual annotations
are no longer needed.)

... because new and deleted annotations don't have IDs,
only tags, assigned by the bridge.

... when hovering over the highlights in the document,
or over the annotation cards themselves.

Consistent names for focusing on annotations.

Remove some dead code

Improve visual diff code

Remove the gray background from annotation cards on hover.

 - Simplify a check for an error condition
 - Add a test case to avoid regressing the same error

As discussed in #1634

When creating a reply, there was an exception
on the console, because the target list is empty,
and some code was not prepared for that.

This takes care of that.

Fix #1661

[WIP] Restore feature to scroll for annotations

Reposition and style show differences checkbox

[WIP] Style the visual diff support

Clicking on an annotation card will bring (one of the) highlights
belonging to that annotation into the view in the document.

Restore visual diff to original behaviour.

... by using ES5 sweetness.

Remove the unnecessary per-target hasDiff flag.

Extract the code for calculating the hasDiff/showDiff flags
into a separate method.

Now there are separate test cases for:
 - A single target with no diff
 - A single target with some diff
 - A single target with some diff, which is case only
 - Multiple targets, some with diffs
 - Multiple targets, some with diffs, but only in case

Furthermore, the used sample diff strings actually look like
real diff strings, for better readibility.

Also, removed some useless lines from the code.

(Thx @aron for all the suggestions.)

Fix holes in message threads

This restores the following corner case:

When there is a difference, but it's only about
the case of the characters, don't want to show
the visual diff by default, but we offer the
feature to the user.

I also updated the tests the reflect the specs.

This will now always keep the branch unless the thread has no
message and no children.

With the messageThread methods. This allows methods to be overridden
by the object itself.

onSetTool calls setVisibleHighlights so this is no longer needed.

- Fix a typo where a closure was unbound
- Always set target.hasDiff rather than having it be sometimes
  undefined and sometimes true.

Thanks @aron