Implement history settings from XEP-0045

[mazhe]

PR check list:

• [X] Document your code
• [X] Add \since QXmpp 1.X, QXMPP_EXPORT
• [X] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [ ] Update doc/doap.xml
• [ ] Add unit tests
• [X] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

Hi, first thanks for your work on the QXmpp project. I am noticing that there is actually no way ton control how much history is send when joining a MUC room. Some clients might want to even disable it to use MAM instead.

I have put together some code to have this ability, basically it add a QXmppMucHistory (not fond of the name, but that's the node name in the spec) element to be used in QXmppPresence and an overloaded version of QXmppMucRoom::join() so that history can, for example, be disabled doing:

QXmppMucRoom *room = manager->addRoom("room@conference.example.com");
room->setNickName("mynick");
QXmppMucHistory history;
history.setMaxchars(0);
room->join(history);

I realize this is not really ideal, this is a bit of a request for comments, I could change it to be stored in QXmppMucRoom if you think this is better.

Also, I don't think there is a MUC testsuite to add some testing? Should I add something in the QXmppPresence testing?