Added folder support

See merge request !11

It will allow smart cache detection «there is a file, so I can get it from cache»
and smart cache date usage «can we use that file? — no, it's too old»
mainly because it will avoid code duplication.
It will also enable better disconnection detection, as we can now handle the
«we got redirected to /enrol» in one place

For now, we keep download_file_async separate, because it avoids keeping the whole file in RAM.

Fixed potential bug with unset application id https://honk.sigxcpu.org/con/GTK__and_the_application_id.html

The moodle provider no longer frees the Courses it gives back, like in e17d48c2

This is to allow GuiCourses to show the content whenever it is needed
instead of instantly