Fix hit testing for background layer

[#57658772]

viewportContaiener was always visible (we were hiding only its
children), so there was no possibility for background container to
pass hit test correctly.
Replace .onClick('plot', ) with 'background' due to class name update

[#57658772]
Repaint canvas when new atom is added or removed

[#57658772]

TODO: think about some consistent, easy patter of canvas update.
When we should redraw it? Which class should manage it?

Calls to modelView.renderCanvas() in various places can quickly
become messy.