Add imnodes widget.

[gucio321]

This is a bit complex widget but I think after a few more redesigns I'll be able to make it useful and easy to use.

Doc

We are 3 things here:

1. NodeEditor which is a workspace for node editor. Its pretty simple for giu impl - we do similar to giu.Plot.
2. NodeWidget coonsists of several things:
   • header (required) (TitleBar)
   • InputAttribute
   • OutputAttrbute
3. Links between input and output elements

My idea is the following usage:

giu.NodeEditor().Nodes(
   giu.Node(1). // maybe this "1" could be of type "comparable" because I'll need to use internal ID anyway
      Static(giu.Label("main content")).
      Output(giu.Label("output")),
   giu.Node(2).
      Input(giu.Label("input")),
   giu.NodeLink(1,2),
)

Note

Also I think that NodeEditor could provide Links method to store lsit of links for reference