Axis label: call immediate base class from keyPressEvent overrides

While EditableAxisLabel doesn't implement keyPressEvent, it is prudent
to not skip base classes as they might do so in the future.

Axivion warns about skipped base classes, and while it doesn't about
those specific cases, fix them anyway.

Task-number: QTBUG-146050
Pick-to: 6.12
Change-Id: I22859531aa4ca6418e594940ebda8a3a1f662a39
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>