Use File Handlers to handle file drag-n-drop in the View 3d. Drop-boxes
still remain since they handle Texture ID drag-n-drop.
This will add-ons to handle drag-n-drop for images and movies while
still providing access to Blender's native support since File Handlers
let users choose which to invoke if there's multiple configured.
Pull Request: https://projects.blender.org/blender/blender/pulls/117728
8b6a21c122