Improve PDF/UA support

[liZe]

Fix #2363.

[liZe]

This branch is ready to be tested.

Some improvements can/will be made, before merging or in a future issue:

• ~warn users when they don’t set a document language or title~ (done),
• ~warn users when they forgot an alt attribute on images~ (done),
• dl elements are poorly tagged, there’s no 1:1 match between HTML and PDF structures,
• PAC complains about some useless spans (just warnings),
• ~hidden titles break levels consistency~ (that’s probably what we want).

Some bugs unrelated to this PR can affect PDF/UA validity, we’ll create new issues for them:

• empty anonymous blocks are created sometimes, for example when list items are displayed as inline,
• a paint operation is done when drawing all SVG tags, even the ones with nothing to draw.

[oraslaci]

I tested the PR on our templates and with @alvarogf97 changes it seems to work almost perfectly (our templates are not perfect either :)

[liZe]

#2482 has been opened to track other issues that are not fixed by this pull request. Time to merge! 🎉