Add `draggable` Key and Access to Child Widget Data Structure in Custom Widgets

Describe the Problem Managing complex widgets like mega menus is difficult: • Each section may have nested items, columns, or tabs. • Widget panel becomes cluttered and confusing. • Elementor provides a data structure for containers, but such access does not exist for custom widgets. Example: A mega menu with three sections (nested items, columns, tabs) in a single widget creates a cluttered panel and complex drag & drop management. Propose a Solution Proposed Solution: 1. Add an official draggable key to control add/remove/reorder in custom containers: "draggable" => [ "add" => false, "remove" => false, "reorder" => false, ] • isDroppingAllowed should respect this key. • This allows locking the container so child widgets cannot be added, removed, or reordered. 2. Provide access to the internal widget data structure similar to Container: • Chil…