Feature: Add relations

## Description

The idea is to link two **Nodes** with a **Relation** between them, so given two nodes in the canvas, you can drag from one node to another to create a relation.

## Behaviors
* [x] draw relations on canvas
* [x] create relation by dragging from one node to another
* [x] do not create relation when dragging from one node to no-nodes
* [x] do not create relation when origin and destination nodes are the same

## Tasks
* [x] fix node refences in relation object
* [ ] fix node finder in relation object
* [x] extract relation object
* [x] return relation objects instead of relation data in `scope.relations`
* [x] add text to relations

## Future
* add from to manually set the text for a new node, new relation
* rotate relation text to path inclination