Added Sticky Notes and their Interaction Logic.

- preliminary implementation of Sticky Notes.
- the GroupInteraction has also been modified to account for Sticky Notes.
- you can now place notes anywhere in your node group, at any group level

- these sticky notes can be resized by dragging any of the edges
  or any of the corner on the sticy note.
- we will eventually add selection of nodes overlapping the sticky note,
  when double clicked.