[BUG #6674] add http(s) support to 'manifest' config entries

- url has either to end in '.json', for a catalog entry, or '.zip/.tar.gz/...',
  for an downloadable archive
- this also fixes bug#6044

- caveat: these changes can no longer be ported backwards to earlier qooxdoo versions,
  by just copying the latest ContribLoader.py to the older tool chain