Markdown: Handle indents when appending or inserting ordered lines

[jeremypw]

Fixes #1475

Based on #1474

This PR ensures that new lines inserted into indented lists are numbered correctly.

Note that it does not globally renumber the whole list as we are not trying to be WYSIWYG. As far as markdown is concerned an ordered list item can start with any number and they will be renumbered correctly when rendered.

[jeremypw]

Converting to draft while waiting for libpeas-2 to be fixed.