DefaultMessageDialog: use gadget StandardButton type

Followup to e17ae3b8b716b00c7cb6156347101f2e789dbd58 : Message.Ok etc.
are no longer available.

Change-Id: I30fc054fc6ace7dc48951739c88f49db2291a7a9
Reviewed-by: BogDan Vatra <bogdan@kde.org>
MessageDialog: handle clicked(button, role) signal properly

Depends on I7be753080794adabb784df9b95ac04aa1c29151c in qtbase.
Now the Android native dialog can provide the same functionality as
the QML and QMessageBox implementations.

Change-Id: Icc9c610669742199d48497096524f6cf8ed4d835
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: BogDan Vatra <bogdan@kde.org>