Don't start QQmlThreads from their constructor

In the case of derived classes (like the only user of QQmlThread) we
cannot guarantee that the class is completely constructed when the
thread starts. This includes the vtable which is necessary to decide
which virtual functions to call.

Change-Id: Ieaea67a72cc3b4f845b8621b34ca2928185917fb
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>