BUG: Fix crash when using slicer dark style

When painting a tab, Cleanlooks style expects to have a gradient in the
Window palette brush when it finds a gradient in the Button palette
brush.
Issue #3251

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22283 3bd1e089-480b-0410-8dfb-8563597acbee