Mirror potentially themeable style hints in QPlatformTheme.

Mirror potentially themeable style hints in QPlatformTheme.

Currently, most of the QStyleHint-values are potentially
configureable by the user in certain desktop environments.

Add them to the QPlatformTheme and query the theme first.

Keep the values in QPlatformIntegration such that simple
integrations that do not implement themes are not forced
to implement them to change the values.

Change-Id: I15742a5968df0ad5d7398cceae640dc7e541da52
Reviewed-by: David Faure <faure@kde.org>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>