[WIP] Add XMPP as second Chat backend

:construction: :construction: WIP -> not ready for testing :construction: :construction: 
cc @jancborchardt @fossxplorer @stefan-niedermann

**Curent state**
Sending an receiving messages for one conversation is working.

**TODO**
- [ ] disbale chat input field when connecting to XMPP server 
- [ ] presence (= offline and online state of a user) of contacts
- [ ] adding the JID of the Current ownCloud user to the Contacts array
- [ ] use the JID's of the contacts instead of a hard coded value
.....