plugin XEP-0447: handle attachment and download: - plugin XEP-0447 can now be used in message attachments and to retrieve an attachment - plugin attach: `attachment` being processed is added to `extra` so the handler can inspect it - plugin attach: `size` is added to attachment - plugin download: a whole attachment dict is now used in `download` and `file_download`/`file_download_complete`. `download_uri` can be used as a shortcut when just a URI is used. In addition to URI scheme handler, whole attachment handlers can now be registered with `register_download_handler` - plugin XEP-0363: `file_http_upload` `XEP-0363_upload_size` triggers have been renamed to `XEP-0363_upload_pre_slot` and is now using a dict with arguments, allowing for the size but also the filename to be modified, which is necessary for encryption (filename may be hidden from URL this way). - plugin XEP-0446: fix wrong element name - plugin XEP-0447: source handler can now be registered (`url-data` is registered by default) - plugin XEP-0447: source parsing has been put in a separated `parse_sources_elt` method, as it may be useful to do it independently (notably with XEP-0448) - plugin XEP-0447: parse received message and complete attachments when suitable - plugin XEP-0447: can now be used with message attachments - plugin XEP-0447: can now be used with attachments download - renamed `options` arguments to `extra` for consistency - some style change (progressive move from legacy camelCase to PEP8 snake_case) - some typing rel 379

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