MD2D renderer: rm unused "atom tooltips" view code

This is not help tooltips, it's the old atom tooltips attached to each atom that were used to scaffold initial development of MD2D and are no longer actually used, at least as of 0.6.0 (PixiJS release).

Probably could implement simpler canvas-based version if we ever choose to reimplement atom tooltips.
delint md2d renderer
Rm extraneous properties from MD2D renderer's api

1. The only thing that consumes the renderer object is common/views/svg-container, aka the very 'modelView' object from which these properties are derived in the first place!

2. The SVG Container never actually reads these properties.

3. Confusingly, there are closure variables with the same name--which are sometimes reassigned!--but these properties don't explicitly reference those variables.
Extract MD2D images handling code to ImagesHelper

[#57495138]

This should introduce no functional changes (functional improvements are what I want to do next). However, it's invaluable for clarifying precisely what parts of the MD2D renderer's internal state are required by the images-rendering code and what parts are not.

The pattern being used to share the MD2D renderer's closure variables with the ImagesHelper may seem a little unusual, but we need a way to share these variables with a friendly helper module by *name* rather than by value. These variables are initialized at different times and can be reassigned to, forcing you to remember to pass the new values to the helper module when they are updated, or to pass them as part of the argument list of every helper module method, or else to risk having stale or undefined values in the helper module.

If the MD2D renderer stored its state as properties of 'this' it would be much easier to share such variables by name with a helper module, and might make it easier to refactor large modules into smaller modules as is done here.
Fix radial bonds midpoint calculation

[#58090240]
Optimize KE shading ("200 atoms with KE shading": 38fps -> 51fps!)

[#57655302]

Note that I get 51fps also for the same test without KE shading. So
now we get KE shading for free.

This commits ensure that we end up only with only two WebGL batches
(one for base atom sprites, second for KE shading sprites). Previously
we had 400 batches, each contained just one atom or KE sprite.
In general PixiJS should handle that, but...

See comments here: https://www.pivotaltracker.com/story/show/57655302
or Pixi issue: https://github.com/GoodBoyDigital/pixi.js/issues/351

Note that we have to ensure that sprites using the same texture are
created together, e.g.:
new PIXI.Sprite(tex1);
new PIXI.Sprite(tex1);
new PIXI.Sprite(tex2);
new PIXI.Sprite(tex2);
...

instead of:

new PIXI.Sprite(tex1);
new PIXI.Sprite(tex2);
new PIXI.Sprite(tex1);
new PIXI.Sprite(tex2);
...

Also, when I add keSprite to atom (using .addChild()) it causes that
I end up with way too many WebGL batches.
rename MD2D ImagesHelper -> ImagesRenderer

[#57495138]
Make ImageRenderer follow sub-renderer patterns

[#57495138]

Still no functional change or significant improvements to the code, but the pattern used to create, initialize, and invoke ImageRenderer is now consistent with the AtomsRenderer, BondRenderer, etc.
Refactor ImagesRenderer to avoid flickering

[#57495138]
Fix CSS pointer events feature test (IE only)

[#59533036]

The class name of SVG element is reported as an object, not string.
It wasn't breaking anything, as the test should report false anyway.
Fix a few issues with mouse events forwarding on IE

[#59533036]
Fix play / pause icon on IE9

[#59563876]
Make fallback color of interactive background more consistent with gradient

It affects IE9 only. Interactive now looks more similar to version
with gradients.
Include Windows in Chrome hairline-bug workaround

[#57658488]
Add .color2number() function to color tools (useful for pixi renderers)

[#58090222]
Add a new benchmark - "200 Atoms with Vectors"

[#58090222]
Render velocity vectors using PixiJS (WIP - no arrowheads yet)

[#58090222]

This is the first approach - using Pixi Graphics API.
Implement velocity vectors using Pixi.Sprite

[#58090222]

They are faster. Of course code will need to be cleaned up when we are
100% sure that it's a better approach. Now it's a useful reference.
Merge pull request #52 from concord-consortium/fixed-image-renderer

Fix flickering of images in md2d

[#57495138]
Fix model-binding bug in MD2D, Energy2D benchmarks

[#59595974]