Closes #5375 and #5377

Similar to #6168, but for footers:

• Table footers can now be placed at any row, not only the last (then, they will repeat up to where they were placed).
• A table can now have more than one footer.
  • Each will repeat from the previous one (or the start of the table) until its actual position, prompting the next one to start repeating.
• A table can now have subfooters: footers with levels. Similarly to subheaders, but in the opposite way:
  • When footers with different levels are placed, they repeat simultaneously.
  • Levels form a hierarchy. When a level 2 subfooter is placed, the next level 2, as well as level 3+, subfooters start repeating, as if it ended the previous "table section".

Implementation details

This will be expanded later. There will still be some significant changes to the code, but here are the most important points:

• Before table layout, we simulate subfooters using an algorithm similar to that of subheader layout, generating a separate "sorted_footers" vector, which contains footers sorted not by their starting row index (which is the normal subfooters vector), but by at which moment they start repeating, to make the layout step easier.
  • This is currently done at resolve, but it could be moved to right before the layout step since this is not used at all during HTML export.
  • Or we could just use some conditional to not do it on non-paged targets.
• On table layout, we start by taking the first batch of repeating footers.
• Widow prevention: The resulting footer height is then subtracted from the available height on every page in order to avoid widows: rows will be placed taking into account existing footers.
• When a footer is placed, it stops repeating, and when we have placed all consecutive footers, we start repeating new footers.
  • This impl still needs to be improved...

Considerations on non-repeating footers

Non-repeating footers are still useful for widow prevention, but for this initial implementation I chose to skip widow prevention for non-repeating footers to focus on repeating footers. Still, a sketch of an idea is to add the non-repeating footer height to the footer height to subtract as soon as we reach the row - or unbreakable group of rows - immediately before the non-repeating footer (or its group of consecutive footers).
Then, it's as if the non-repeating footer's height became part of the preceding row's height, ensuring both fit on the same page.

TODO

• Gutter at the top of the footer: it is repeated, though there is a problem when you have a header and a footer one after the other: the gutter row will be part of both at the same time, which is weird. Not sure if we should consider this strictly a problem, and we may want to just ignore it, since this is a layout detail.
  • This could generate a problem where the same gutter row could be placed twice in a row, but that was fixed and now the remaining question is purely cosmetic I think.
• Improve widow prevention impl. In particular we may want to use a "pending footers" approach similar to headers, which would also generalize well to non-repeating footers: we dont stop repeating the current footers when we find them until we place something before the next footers.
  • The current impl stops repeating them immediately, and seems to cause some problems, generating a widow even in a simple demo.
  • However, I wouldn't consider this a blocker for releasing this feature as it's already functional, though it produces some non-ideal output at the moment.
• Distinguish non-repeated from short-lived footers (footers that might be repeatable, but they would just stop repeating immediately, so why bother).
  • Also not strictly necessary for an initial release.
• Footers are being cloned all around... can we do better?