Allow hiding all headings in manuscript

[vkbo]

Summary:

This PR changes the way hidden headings work in Manuscript builds and adds a hide button for all of them. The hide effect is also processed first before formatting, instead of after where the hide only applied to empty formats.

Scene separators are also skipped from the start of the document build, not only after the first level 1 or 2 heading.

In addition, this PR improves how empty paragraphs are formatted in Extended Markdown output.

Related Issue(s):

Closes #1756

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed