Annotation mouse events

[kaglowka]

To be merged after #357

Preparing for #351 (I just reopened it since it was not fully finished...)

Clearer division of responsibilities:

• highlight-manager only sets the viewer window open or reopens it when hovering over other highlights
• Viewer sets redux mouseOver state
• ViewerManager tracks changes in redux state and decides whether to close the window

This is meant as a solid base from which annotationRequest highlighting (almost trivial compared to Viewer) can be replicated.

[glowka]

I rebased it on current master

[kaglowka]

Hard for me to judge the details, but I will always support spiting responsibilities ;)

:) One more detail I forgot -- now viewer mouseOver state is only based on timer hover checking (rather than mouseenter / mousleave events) This also removes redundancy.

At first I tried a converse approach, i.e. using only DOM mouseenter / mouseleave events, which was more or less successful except for viewer non-DOM children that were mounted after Viewer first render. It was tempting, since it didn't require timers, but all in all timer-based hover checks are probably more robust.