…sier to understand when interface `Paragraph<S, PS>` is implemented .

…NormalParagraph will implement.

…graph to Paragraph.

EditableStyledDocument details
- Constructor now initializes itself with an EmptyParagraph
- Replace method checks for NormalParagraphs that should be EmptyParagraphs because their length == 0
- getStyleForInsertionAt(Position) will return `initialStyle` if that boolean property is true or if the Paragraph returned is EmptyParagraph

ReadOnlyStyledDocument details
- The following were adjusted to account for saving/loading an EmptyParagraph or NormalParagraph:
 - fromString() method
 - Codecs

…om NormalParagraph to Paragraph

ParagraphText details
- When ParagraphText is displaying an EmptyParagraph, it will seem like the paragraph and the caret aren't visible. To fix this, an empty TextExt was added. There is probably a better way to make the line and caret appear. Additionally, the caret's height might not appear to be the same elsewhere in the area if the initialStyle used in EditableStyledDocument doesn't specify what the fontsize should be of the text.
- There's probably now a way to show that a newline char is highlighted when user selects something by `new TextExt(" ")` rather than `new TextExt("")`. For example, when a user selects multiple lines and one of them is an EmptyParagraph, the TextExt could be used to show that that line is selected. As of now, there is no indication of this to the user.

…o that `StyledDocument`s that are used to initialize a new `RichTextChange` don't have any `NormalParagraph` instances in their respective paragraphs when those instances length is 0. Instead, remaps the paragraphs to `EmptyParagraph` objects using `EditableStyledDocument`'s `initialStyle` and `initialParagraphStyle`.