Enable additional content inside Form/Section/Group

[aalmiray]

Fix for #23

WORK IN PROGRESS!!

This PR adds a new base type Element that can identify regular Fields and plain content (Node).
Instances of NodeElement can be used to attach arbitrary content to a Group within the boundaries of its grid, as shown by the following screenshot

The code added to the end of the Section definition is

NodeElement.of(new javafx.scene.control.Label("Somebody setup us the bomb"))
                               .span(ColSpan.HALF),
NodeElement.of(new javafx.scene.control.Label("All your forms are belong to us. What you say? Move every zig. For great justice!"))

I'm not particularly fond on the current implementation of GroupRendererBase to identify how a particular element should be rendered, as it can only identify Field and NodeElement.

Feedback welcome.