Preserve the text style when splitting at the end of paragraph.

At the moment, splitting at the end of the following paragraph
<p style-name="standard">hello <span style-name="bold">world|</span></p>
will create a new paragraph like below
<p style-name="standard">|<span style-name="bold"></span></p>

The cursor is moved out of the span due to empty span is not considered as
a valid cursor position in the current design. this causes newly entered
text are not formatted correctly.

Merged DirectTextStyler and DirectParagraphStyler into
DirectFormattingController. These have become closely related concepts as
a result of changes in this patch.
Pass down the directParagraphStylingEnabled flag to DirectFormattingController.js, disable the paragraph related methods if the flag is true.