Always use QNetworkAccessManager, including for local resources

Should allow to benefit from caching.

In this case we should be able to add the missing glyphicons-halflings-regular.ttf file to the fonts directory allowing to:

• Get rid of the JS error complaining about this missing file.
• Have a good display of the font under Linux systems that don't manage to load it without the ttf file.

As ttf files breaks display under OS X we'll still need to filter them but for Mac only, see https://github.com/nuxeo/nuxeo-drive/commit/6c054e3f1487ab14811beb970d3a84756fb8c419