Node editor demo expanded

[hwps]

This PR expands on the node editor demo to make it a functional example of a node-based UI.

Features:

• multiple node types
• multiple connector types
• linking between connectors of the same type
• detaching and reattaching links
• getting value from linked node if slot is connected

Some notes:

• only tested with the glfw_opengl3 backend
• uses system malloc, not sure if nk_malloc should be preferred.
• uses the standard panel close button , then checks for NK_WINDOW_HIDDEN whether to delete a node or display it. Not sure if there's some pitfall here.

[leobenaducci]

Nice stuff

[RobLoach]

I like where this is going. I'm not sure what's wrong with the demo though? This is the sdl_renderer....

[hwps]

@RobLoach Can you click the connectors to draw a link or right click to add nodes?

Maybe there should be some more nodes or at least links in the initial state. I realize now that for someone not used to a node-based UI it might be non-obvious what to do.

[hwps]

Ok, finally did this slight update to the init state. Screenshot attached.

[riri]

Nice work! I ran it on glfw_opengl3 and x11_xft demos (screenshot for the later below)