Snapping for Gradient gizmo

Along with various other snapping improvements it would be good to have snapping Gradient gizmo handles to object origin, midpoints, path elements and bounding box.

<img width="1062" height="1004" alt="Image" src="https://github.com/user-attachments/assets/f313a7c0-f852-4fc8-b089-3abc8053fe4f" />

original: https://github.com/GraphiteEditor/Graphite/issues/3261