Crash in case of read-only config

[nico-izo]

If ~/.config/qutim/config/profile.json is read-only, qutIM will crash during start

Q_ASSERT here: https://github.com/euroelessar/qutim/blob/master/src/lib/qutim/config.cpp#L1354

	Q_ASSERT(atom->isMap() && !atom->isReadOnly());

We probably should either print nice message or have in-memory config without being able to save it.