Consolidate StandardButton, ButtonRole and related functions

Moving them into QPlatformDialogHelper for the convenience of both
widgets and QtQuick.Dialogs.  The main reason is to ensure that
QtQuick.Dialogs does not need to depend on the widgets module, in
order to re-implement the button box concept in a generic dialog.

Change-Id: I471dff185767d05f3579c76082550008085ee681
Reviewed-by: Liang Qi <liang.qi@digia.com>