Implement OpMergeParagraph

In order to improve the OT-ability of text and paragraph removal
with other operations, the merge behaviour has been extracted.

The logic for building up the necessary remove operations has
been encapsulated in TextController for now as all uses pass
through this point still.
Cleanup leading/trailing insignificant whitespace on paragraph merge

Failing to clean up these empty whitespace blocks will cause them to
unexpectedly become significant whitespace after a merge is performed.