Fix qutepart crash

This provides a Python-invoked finalizer. The C++ destructor doesn't seem to be invoked, hence connecting to qpart.destroyed() never gets invoked. Perhaps this is because of placing core = Core() in Enki's core.py, preventing it from being destroyed?

Note: this also requires adding the statement `self.qutepart.del_()` at line 228 of Enki's document.py.