Fix splitting in the middle of a text block

[nylen]

As a follow-up to #409, this fixes an issue with splitting a text block in the middle of its content:

• Create a text block with two paragraphs of content
• Move the cursor to the end of the first paragraph
• Press Enter. The text block will be split into two.

Instead, I'd expect that two Enter presses would be required to split the text block in the middle of its content, like when we edit at the end of a block.

The reason for this issue was that TinyMCE inserts a <br data-mce-bogus=1> element inside of our new second paragraph, and this causes the splitIndex to be -1 because we should really be looking for the parent p element of the br. The solution I found is to walk the DOM tree until we find the currently selected top-level element of the block (in this case the p element).

[nylen]

Also, how can we add tests for this fairly tricky bit of logic?

[youknowriad]

This works great! Thanks for the fix (I had some trouble understanding the issue at first)

Yeah this code is tricky and hard to test. Maybe we could just extract to a splitNode( contentNode, selectedNode ) which returns { before, after } or false.