tdf#94112 KDE4 use disabled WindowText color

Libre office just has a general disabled color. Instead of the
general QPalette::Mid, use the color QPalette::WindowText of
the palette QPalette::Disabled.

Change-Id: Ie6943b455d3e6545289b241ee4ab85128f4fd7ab

vcl/unx/kde4/KDESalFrame.cxx

@@ -243,7 +243,7 @@ void KDESalFrame::UpdateSettings( AllSettings& rSettings )
     style.SetTabHighlightTextColor( aButn );
 
     // Disable color
-    style.SetDisableColor( aMid );
+    style.SetDisableColor( toColor( pal.color( QPalette::Disabled, QPalette::WindowText ) ) );
 
     // Workspace
     style.SetWorkspaceColor( aMid );