Use `FileHandlers` to handle file drag-n-drop in Node Editors. Drop-boxes
still remain since they handle Images ID drag-n-drop.
This also allows to open/drag-n-drop multiple files at once.
Also this will allow add-ons to also support drag-n-drop for images and
movies in node editors 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/121051
615100acda