Fix pageBreakBefore with unbreakable blocks

[rstrand]

I was trying to implement the documented orphaned-children pattern to prevent headings from being stranded at the bottom of a page:

pageBreakBefore: function(currentNode, nodeContainer) {
  return currentNode.headlineLevel === 1 &&
         nodeContainer.getFollowingNodesOnPage().length === 0;
}

This didn't work when the content following the heading was inside an unbreakable: true block. The callback's getFollowingNodesOnPage() was returning the unbreakable block's nodes as still on the same page as the heading, even though they had moved to the next page. I also found that header/footer nodes were appearing in the results, further preventing the length === 0 check from ever being true.

I tracked the cause down to a few issues:

• _node back-references, which addFragment() uses to update positions when blocks move pages, were only set on lines with tocItem or id. I changed this to set _node on all text lines and added tracking for image, SVG, canvas, QR, and attachment nodes.

• Nodes from dynamic headers and footers were being pushed into linearNodeList, which contaminated getFollowingNodesOnPage() and the other pageBreakBefore helpers with irrelevant nodes. I added a flag to suppress this during addDynamicRepeatable() processing.

• I also noticed that attachments inside unbreakable blocks were being silently dropped because addFragment() had no case for the 'attachment' item type.

This PR addresses these and adds relevant test cases.