It is implemented using GSettings' bind. Other parts of the code are to
be adapted to use it.

Closes #24

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.