Generate `simpara` for `ordered_list` and `itemized_list`.

Also fixes block elements nested in lists.
Fix single entity source mode notes.
Remove incorrect note about document id.

The document id has never been generated from the filename. It comes
from the docinfo `id` attribute, or if that doesn't exist, it's
generated from the document title. Although, thanks to bugs, it's a bit
more complicated than that.
Stop saving phrase and block output separately.
Make list generation state part of the output state.

Which sounds obvious when it's put like that. But I originally did this
in the parser because the parser was tracking lists anyway. But that
made a mess of handling `[ordered_list]` and `[itemized_list]`, I tried
fixing that but made a pig's ear of it.

The code for handling 'explicit lists' will still be a bit messy because
the generator is currently oblivious to what type of block element it's
generating markup for, and it needs to be aware for lists.
More consistent markup for lists in templates in lists.
Fix bug where list tags were confused with markup lists.
Revert and clean up some of the list changes.

Now that I've fixed my mistakes I can clean up a lot of the rubbish.
Hard-code which collector is written to.
Add note about hack.
Simpler, less efficient, id generation.

I noticed a bug for an edge case in the id generator. Wouldn't be too
hard to fix, but the implementation was too complicated with some really
pointless optimizations, so I rewrote it using a simpler brute force
method. Will be fine for now unless something has a lot of duplicate ids.
Quickbook 1.6 alpha 1.
Start to document quickbook 1.6 include.
Fix inaccurate heading id generation documentation.

All headings have ids generated, not just `h1`, `h2` and `h3`.
Update the import and include documentation.

This is a bit incoherent at the moment. It needs to be rearranged so
that there's a tutorial that introduces the concepts in a more logical
order. But I want to press on with some of the other updates.
Update version meta-data documentation.
Add explicit ids to heading documentation.
Add note about xmlbase.
Add new elements in 1.6 to the main documentation.