Add further theme hints to QPlatformTheme.

Add further theme hints to QPlatformTheme.

- Add hints for QDialog/QDialogButtonBox.
- Add hint for available popup-menu area.
- Add keyboard scheme hint replacing
  QGuiApplicationPrivate::currentKeyPlatform()

Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>
Task-number: QTBUG-24204
Task-number: QTBUG-24315

Change-Id: I6653786b0dcb49a6fc264d3b9891dbfee502bd3e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>