[WIP][ticket/11915] Improve Plupload integration

Depends on https://github.com/phpbb/phpbb/pull/1805
Known problems:
- Some files will fail if you add more files to an already running queue. The files seem to upload successfully, but Plupload seems to not detect for some unknown reason.
- Need to check for trigger_error() being called after sending a chunk.
- Some strings are not translated a the moment.

http://tracker.phpbb.com/browse/PHPBB3-11915
http://tracker.phpbb.com/browse/PHPBB3-11961