Fix TLW destruction after user closes it (not done by Qt)

Note that Qt don't destroy the main window on close by default (except Qt::WA_DeleteOnClose is set).
So, if Dialogs could remain alive (hidden) if user closes the window and the program don't call explicitelly to Destroy() / delete.
This was causing a segmentation fault as when the apps finishes, the Top Level Windows where destroyed in order (and then a child dialog was being destroyed after its parent).
The issue was reproducible easily with the combo sample, if the "Compare Dialog" was open using the menu, as the code was not calling Destroy nor deleting it explicitelly/implicitelly.