Add OT for InsertText and SplitParagraph at the same position.

The person typing text should keep typing in the same line
without any unexpected jumps.
The person splitting the paragraph should get that without having
the other typing in the new one.

Also added tests for all 3 position cases (before, same, afteR)
in InsertTextSplitParagraph.
Merge pull request #546 from adityab/split-insert-ot

Add OT for InsertText and SplitParagraph at the same position.
Add core.UnitTest.createXmlDocument, to reduce duplicated code