- This fixes memory leaks on unix systems with VirtualGL.
- With VirtualGL (vglrun) hidden widgets/windows are not
correctly handled. So with the previous implementation,
`Finalize` is never called and there are several leaks reported
upon exiting the python interpreter.
- Instead, connect the parent's destroyed signal to
widget's close slot. Qt does not close child when parent
is closed.
- closeEvent is already implemented to call Finalize.