Improved Filter tool UI by fixing the position of Blending Options
The current UI of the Filter tool behaves like this:
User has to move their mouse again to show/hide the Blending Options.
With this patch, the filter tool behaves like this:
The Blending Options expander is fixed to its position. This has been achieved by making the scrolled window in Operation tool un-expandable, and packing the Blending Options box from start instead of the end.