Fix GTK4 filter primitive reordering

Yuktarequested to merge
XYuktaaa/inkscape:fix-filter-primitive-dnd into master

Fixes #6008

Fix filter primitive drag-and-drop reordering regression in GTK4 Filter Editor

Summary

This MR fixes a regression in the Filter Editor where drag-and-drop reordering of filter primitives visually worked but did not update the underlying SVG filter primitive order after the GTK3 → GTK4 migration.

As a result, reordered primitives would revert or have no effect on rendering, unlike the correct behavior in Inkscape 1.4.x.

Problem

Drag-and-drop for reordering filter primitives doesn't work. Custom DragSource conflicts with set_reorderable(true), causing the on_drag_end() handler to never fire.

Solution

  • Remove broken custom DragSource
  • Use signal_row_deleted() + idle callback to update XML after GTK finishes
  • Prevents iterator invalidation crashes

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