Smarter rebuild on content change

[Keats]

I'm currently leaving TODOs all over the place so the rebuild_after_content_change will take as little time as possible.
Do it once the new way of handling index is in

[Keats]

#62 is a first step to make things faster when editing the content of a page.

There are a few other situations:

• editing front matter of a page: need to check what changed to know what to re-render but even then since people can put whatever in extra you're never sure what should be changed. Might be safer to just re-render the whole section if there is one, tags/categories if they changed and index.

• deleting a page: should re-render the section if there is one, index page and tags/categories if there were some

• deleting a section: should re-sort/set prev&next to pages int the section and re-render index

Need to see what happens in terms of event when doing bulk actions, like renaming a folder for example.