Clean up QPlatformSessionManager

Remove virtual from appCommitData and appSaveData and make them public

Change-Id: Ifd605742c6552b7a407de9bf482f8f0f0aa125ca
Reviewed-by: David Faure <david.faure@kdab.com>
Clean up QXCBSessionManager

Removed useless overrides and inline simple functions.

Change-Id: I69df0ec8446fe2c8bb904c7d4d9cc2e1d269533b
Reviewed-by: Teo Mrnjavac <teo@kde.org>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>