Further progress in counter editing in the plugin config

-) Implement loading indicator during the AJAX calls
-) Give each type of number its own fieldset (and thus its own
   tab in the plugin config)
-) Fix display of system messages generated by ajax call in Joomla 2.x
-) Fix JSON output in Joomla 2.x (two Content-Disposition
   headers generated, which broke parsing of the AJAX response
   in Chromium) -> Use raw format for joomla and manually send
   the json content-disposition header
-) Fix jQuery in Joomla 2.x