Expose a "constructor" for EditableStyledDocumentImpl in its interface

[JordanMartinez]

Slightly related to #278, but could we add a static method to EditableStyledDocument that returns back an instance of EditableStyledDocumentImpl?

In my project, I'm creating the model first and then passing that to the view to display. My view has multiple components to it, one of which is a StyledTextArea. Since I don't have my own custom implementation of EditableStyledDocument, I currently have to create a fake StyledTextArea just to get an instance of the implementation class via StyledTextArea#getContent. Once the model has been fully constructed, I then pass it to the view which creates one or more areas that share that document.

[TomasMikula]

Being able to create an EditableStyledDocument without creating a StyledTextArea sounds reasonable. Maybe I would even make the class public, but rename it to e.g. SimpleEditableStyledDocument.

[JordanMartinez]

Why make it public though? You seemed to be against that beforehand.

[TomasMikula]

I think I was mainly against it serving as a base class for custom EditableStyledDocument implementations that users would inherit from. I still hold that position, and the class is indeed final.

[JordanMartinez]

Resolved by #309