Need to investigate on an abstract superclass for BlElement

[plantec]

Having a basic BlElement could be beneficial for memory usage and also for time consumption:

• To have a video element based on GStream that doesn't need to consider borders, etc... just a rectangle where low-level will render there a stream.
• BlTextElement may not need all what inherites from BlElement. Just render text and no background, border, outskirts, etc.
• image with Form
• Image with SVG ... right now all internal groups of a SVG are imported as a new BlElement and each of those SVG parts will have initialized event handlers, outskirts, etc, etc.

[plantec]

related to #678

[tinchodias]

The Node class hierarchy of JavaFX8 can help us to think on this issue:

https://docs.oracle.com/javase/8/javafx/api/javafx/scene/Node.html

[tinchodias]

Our BlElement partially matches Shape because it has transformation, stroke and fill. But we have a geometry as a collaborator object while they model the geometries as Shape subclasses.