* Avoid duplicated setLastNode(..) call

The call was happening first irrespective of whether the node was a TextContainer so lifted this logic out of the `if` block.

* Avoid duplicate LineBreakNode logic

It seems that currently LineBreakNode is independant of TextContainer so it seems safe to pull that out of the `if` block.

* Paragraph with trailing blank line collects trailing blank line

Consequently only a single new line is needed to add paragraph breaks

* Collect a blank line before all (except first) Block instances

- Removed special case logic for Paragraph blocks
- Generalised from BlankLineBreakNode case

* IndentedCodeBlock implements TextContainer

Implementation based on Text collectText(..) but using getContentChars()