Uses `FileHandlers` to handle file drag-n-drop in the space Sequencer, drop-boxes
still remain since they handle Images ID drag-n-drop.
This also allows to open/drag-n-drop multiple files at once.
When drag-and-drop is performed with a single image/movie/sound
file and only the internal `FileHandler` can be used, is used
the internal drop-box api for drag and drop, since it has a nice
drag-and-drop preview.
Pull Request: https://projects.blender.org/blender/blender/pulls/119927
89c1f7e0c3