Make QKdeTheme constructor public, allowing for custom inheritance

Make QKdeTheme constructor public, allowing for custom inheritance by
other classes. The danger of making it public and not going through the
factory method are close to none, while other developers might want to
slightly enhance KDE-themed platform themes by themselves. Right now
the only choice is to copy-paste the whole code. QKdeTheme might be a
private class, but still it makes no sense to restrict derivation if
someone finds it necessary to do so.

Task-number: QTBUG-35566
Change-Id: Ia3488da71e7c06fe4b7dc6e275605970b435b00e
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>