We don't have any delay in between updates (because i'm scared of
threading), which has a few problem:
1. probably really sluggish on slow systems
2. probably slow for readmes with images (need to test)
3. flickers (the phantoms are updated too quickly, so sometimes it
doesn't replace the old one smoothly)
BUG: the preview doesn't load when we preview the markdown file
It use to ask for confirmation (to save) because on_pre_close is run
after this dialog. Hence, we set the markdown_view as scratch for
unsaved files. :^)
It works for saved and unsaved files. Maybe unsaved file's content
should be written to a temporary file in case we crash, so that the user
doesn't lose all it's content.
