make kitchen-sink-accessible example fully PDF/UA compliant

[andreiaugustin]

Bug fix

What kind of change does this PR introduce?

This PR makes the kitchen-sink-accessible example fully PDF/UA compliant, fixing #1633.

There were a few issues:

CS operator violation (ISO 32000-2 Fig. 9) - essentially color space operators aren't permitted during path construction, so any fillColor or strokeColor must happen beforehand. I looked into the possibility of caching things so at the end we can write them to the PDF in the right order, but it's not as straightforward as I would've hoped and I think keeping things simple without changing too much might be best, at least for now. Perhaps we should document this, if it is not already. Edit: added to docs within this PR.

The figure structure element required a bounding box, so a bbox option was added to PDFStructureElement which writes out a BBox entry into the 'Layout' attribute. Also, PAC was still throwing errors about improper use because we were missing the Placement attribute which is now added and defaulted to Block, but can be overridden with the placement option.

The Link needed to be contained within a block-level element, so I've wrapped it in a P in the example. ISO 32000-2 Annex A says it's an inline-level element so needs to be wrapped into a P or H1 for example when inside a grouping element like Sect.

Now, there's another issue which is not directly fixed here, but I think it cropped up in other issues already: the built-in fonts, being AFM-only, they don't embed and that is required in order to be compliant with PDF/A or PDF/UA, so using the built-in fonts will result in a non-compliant PDF output. Short of growing the size of the library with full ttf built-in fonts which can be embedded. , I reckon we should just document this as a limitation/requirements if anybody wants to produce compliant PDF/A / PDF/UA? Edit: added to docs within this PR.

Checklist:

• Unit Tests
• Documentation
• Update CHANGELOG.md
• Ready to be merged