Fix backingstore crash with QAxWidgets

Fix backingstore crash with QAxWidgets

The crash was caused by the fact that backingStore parameter is always
null (there seems to be no call in codebase that uses anything else but
the default values for this function).
Using "store" member variable instead of "backingStore" parameter gets
rid of the crash, and it is how it was in Qt4 - probably the bug crept
in when the paremeter and member variables were renamed in Qt5.

Task-number: QTBUG-25803
Change-Id: I4b1ccf540fddd6baa1dffa7f8165272b54caf238
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>