QPlatformDialogHelpers: Reduce dependency on QDialog.

QPlatformDialogHelpers: Reduce dependency on QDialog.

For each QDialog-derived class, introduce a Q[X]Options class
containing the options of the dialog. An instance is shared
between the QDialog (or dialog desktop component) and the
helper.

Change-Id: Ibabf508a4b9eaea25615638a47a4c1b8f93c019e
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>