More-understandable names for svg container parts

A start on [#57495348]
rename MD2D "mainContainer" -> "atomsContainer"

[#57495348]

This clarifies its role.
Allow dragging the svg container's <div> not <svg>

[#57495348]

Since we'll split the single <svg> into several, we need to make sure dragging works.
Let renderers ask container to make a "viewport"

[#57495348]

Also, create 3 separate viewports in MD2D renderer
Restructure svg container into separate svgs

[#57495348]
Fix gridline rendering in separate-rendering-layers

[#57495348]
Fix genetics renderer in separate-rendering-layers

[#57495348]
Simulate SVG hit testing for separate-layers

[#57495348]

i.e., clicking now works (no touch events though)
Merge branch 'separate-rendering-layers' into 'pixijs-rendering'