Fix OT when two adjacent paragraphs are merged backwards

Transforms were not ensuring a consistent style name was applied to the merged paragraph on all clients.

Incidentally... this issue was found when playing with the split screen editor!
Let's walk through the test:

`<office:text><text:p>a</text:p><text:p text:style-name="b">b</text:p><text:p text:style-name="c">c</text:p></office:text>`

Operations:

`<op optype="MergeParagraph" memberid="Alice" destinationStartPosition="2" sourceStartPosition="4" paragraphStyleName="c"/>`
`<op optype="MergeParagraph" memberid="Bob" destinationStartPosition="0" sourceStartPosition="2" paragraphStyleName="b"/>`

This matches the condition of this block.

According to the transform logic, the new operations are:

`<op optype="MergeParagraph" memberid="Alice" destinationStartPosition="0" sourceStartPosition="4" paragraphStyleName="b"/>`
`<op optype="MergeParagraph" memberid="Bob" destinationStartPosition="0" sourceStartPosition="2" paragraphStyleName="b"/>`

The modified operation of `"Alice"` will be executed on `"Bob"`'s client, which has already executed it's own operation.

So, the xml fragement is:

`<office:text><text:p text:style-name="b">ab</text:p><text:p text:style-name="c">c</text:p></office:text>`

onto which must be executed

`<op optype="MergeParagraph" memberid="Alice" destinationStartPosition="0" sourceStartPosition="4" paragraphStyleName="b"/>`

But there is now no paragraph with source position `4`.
Or am I going totally wrong somewhere? :confused: