WIP port: re-add our label rendering as a method override (instead of inline like in sigma.v1.customized.js)

WIP port: new sigma allows a simpler click one node -> MultipleSelection (no need for selectionEngine at all for one node)

WIP port: change most frequently used accessors (nodes/edges[i]), rename internal customAttrs for clarity, fix settings syntax and color logic

This reverts commit 4d5fba0b.