domish.Element.elements should be called with namespace and element name as 2 arguments,
but it has been confused in several places with the call to `domish.Element.addElement`
which is often done with a `(namespace, name)` tuple. Unfortunately calling with a tuple
is accepted and doesn't raise any error in `elements`, but this result in a wrong element
returned.
This patch fixes the erroneous calls.
attachment handling has been simplified, and now use a "register" method similar as the
ones used for download or upload.
A default method (for unencrypted messages) will try a simple upload and will copy the
links to body.
AESGCM plugin has been adapted to be used for encrypted files. If more than one file is
sent with AESGCM plugin, they will be split in several messages as current de-facto
standard (OMEMO media sharing) doesn't support several files per message.
SàT was using 16 bytes IV when sending files withr AES-GCM, due to ChatSecure being only
compatible with that. Monal, an other iOS client is only compatible with 12 bytes IV, and
ChatSecure has fixed its code to also handle 12 bytes IV, so there is not reason anymore
to use 16 bytes, and SàT now uses 12 bytes.
The scheme is register with download meta plugin, and it is activated with HTTP Upload
(XEP-0363) when `encryption` is used in `options` with the value of C.ENC_AES_GCM.
This is also known as `OMEMO Media Sharing` even if this method is not directly linked to
OMEMO and can be used independently.
