Names of the initGL and renderGL signals have been changed to
initializeGL and paintGL in order to unify the naming of the API methods
with QOpenGLWidget component.

Change-Id: I074a7609347a3297dabcc4295fb92b9193cf00ed
Task-number: QTBUG-45665
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Titta Heikkala <titta.heikkala@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@theqtcompany.com>