Added RNA for the unified paint setting flags that matches the Brush RNA. Fixed the getter/setter functions to avoid guessing which Scene's UnifiedPaintSetting to use. The getter functions take a Scene pointer now, the setter functions are removed in favor of a more explicit approach through RNA: Rather than RNA choosing whether a property's value is in the Brush or in the UnifiedPaintSettings, there are now explicit properties for both. The UI code has been modified accordingly to switch the toggle buttons between affecting the Brush and the UnifiedPaintSettings.