Make file filters asynchronous and remove BeforeAdd event.

BeforeAdd was meant to mimic BeforeUploaded potential, but it failed,
since filter operations by their nature might be asynchronous and this
successfully makes BeforeAdd completely useless. Since while UploadFile
can be canceled from BeforeUpload and then invoked manually when
required, there's no such mechanism for BeforeAdd and FilesAdded. For
now plupload.addFileFilter with a callback as a third argument seems
enough.