all node editor tasks

[JackDotJS]

• allow creation of new node connections
• prevent linking nodes to themselves
• find a way to deal with z-index for both nodes and node connections
• allow creation of new nodes (node menu or context menu)
• allow deletion of existing nodes (delete key or context menu)
• allow renaming of nodes (context menu)
• add further limits on position to prevent nodes from going right up to the edge of the grid
  • should help with minor overflow issues
• prevent nodes from (visually) going more than 50% beyond the bounds of the editor viewport
  • only really needed for UX, allows users to see what node they're dragging around without having to stop scrolling
• add zoom control
  • this one will be difficult. i'm still not entirely sure how we're gonna deal with this one. css transform would be perfect, but they don't affect scroll bar behavior, which is a significant issue. will need more research and experimentation here
• add selection controls