Move attachments with the keyboard

This will be great for both accessibility and power users.

We can already select images with the keyboard and mouse. It would be a matter of adding keyboard support. I propose `ALT + ARROW KEY` to move selected attachments:

1. Move the attachment before the previous block.
2. Move the attachment after the previous block.
3. When there is an image in the destination position: create a gallery.
4. When the selected image is in a gallery: UP and DOWN will let you move it out. LEFT and RIGHT will reposition the image in the gallery.

@brunoprietog I would appreciate your input here.

cc @zoltanhosszu @samuelpecher

Begin by tracing the existing keyboard and mouse selection flow for images and the attachment/gallery behavior. Verify the proposed ALT+ARROW rules for block moves, gallery creation, and moving images into or out of galleries; done means each listed operation works consistently from keyboard input.

Written by the indexing model from the issue text.