Use QXmlStreamReader internally

[lnjX]

• StreamOpen: Impl fromXml() using QXmlStreamReader
• XmppSocket: Parse whole stream through QXmlStreamReader

PR check list:

• [ ] Document your code
• [ ] Add \since QXmpp 1.X, QXMPP_EXPORT
• [ ] Update doc/doap.xml
• [ ] Add unit tests
• [ ] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>