QML plugin registration isn't thread-safe

In qqmlimport.cpp we frequently access the global qmlEnginePluginsWithRegisteredTypes(). That's a Q_GLOBAL_STATIC, so its retrieval is actually thread-safe. However, when we call methods on it we still have to protect it from concurrent access. As it stands, when starting multiple QML engines in parallel you can easily crash the application by concurrently importing things.