Be (more) correct about the widgets and parts, we actually can draw
natively in KDE4.

Also add an assert, so drawNativeControl won't be called with values
not supported by IsNativeControlSupported.

Change-Id: Iac6c78194191a9cd059ace560c285263ce7c1045
(cherry picked from commit 88e16529)
Reviewed-on: https://gerrit.libreoffice.org/16899Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Tested-by: Katarina Behrens <Katarina.Behrens@cib.de>