Ensure OpSetParagraphStyle position is first step within the paragraph

The merge paragraph op depends on the impacted paragraphs being identified
by the position of the first step within the paragraph. In order to
facilitate OT'ing of paragraph-based ops, all paragraph ops need to
consistently use the first step in the paragraph to identify the paragraph
they impact.
Make SplitParagraph op specify the new paragraph style

In order to allow convergence between collaborative clients, the split
paragraph op needs to specify what the resulting style of the new
paragraph will be.
Remove createOp function to improve closure compiler's type checking

Having the createOp function prevents closure compiler from accurately
detecting if any of these ops are being constructed with incomplete or
incorrect data.
Implement OT for SetParagraphStyle vs. SplitParagraph