[preview] support "stickers" sent by Movim (XEP-0231: Bits of Binary, BoB)
Movim sends e.g.
<!-- Incoming 2018-02-26T08:08:08 CET -->
<message type='chat' to='gajimuser@myserver' from='movimuser@movimserver/9876543210123456789876543210' id='12345678-1234-1234-1234-123456789abc' xml:lang='en'>
<html xmlns='http://jabber.org/protocol/xhtml-im'>
<body xmlns='http://www.w3.org/1999/xhtml'>
<p>
<img alt='Sticker' src='cid:sha1+1234567812345678123456781234567812345678@bob.xmpp.org'/>
</p>
</body>
</html>
<active xmlns='http://jabber.org/protocol/chatstates'/>
<request xmlns='urn:xmpp:receipts'/>
<markable xmlns='urn:xmpp:chat-markers:0'/>
<body>A sticker has been sent using Movim</body>
<stanza-id id='0123456789abcdef' by='gajimuser@myserver' xmlns='urn:xmpp:sid:0'/>
</message>
The plugin should request the sticker, if not yet cached, and then receive, display and cache it.
Currently, the body text is shown instead: "A sticker has been sent using Movim"