Unit parser - move images to standalone section

[IanMayo]

I'm looking at unit_delta12.dita

The first section in the body is the signatures table. It starts with a div of images. This was pulled from lower down in the signatures block.

We can/could better exploit the images (including potentially having a gallery component) if they were moved out of the signatures section, into an images section.

Look at patterns in the target data for these images, and ensure we represent it in the mock data. Then work on putting images into their own section.

Note: back when we worked with custom DTDs for our document types we did have an image-list element defined for unit/class/installation documents. We're using a named section as an alternative to this.

[IanMayo]

This is related to #606

[IanMayo]

This is working. We're also applying an imageList class to the div that contains them.