Add node context menu + functionality

[gk646]

This is a bigger task but consists of smaller parts:

• Create a context menu when you right click on nodes

  • Normal context menu only opens when you right click the canvas
  • Should show option to: Delete Node, Copy Node, Cut Node, Fold Node

• Implement onHover button for nodes in the top left corner to collapse them into their header only

  • Connections are drawn as starting from inside the header
  • Use the header color