document in the background and magnets should be the same thing

[francoisschwarzentruber]

Since magnets can be put to the background, there is no need anymore to make a difference between document in the background and magnets.
Maybe we keep it in the menu, but technically it should be the same.

Pressing b on a magnet pushes the magnet in the background
and pressing b on a "document" should put that document in the front (so that the user can move it and push back to the background again for instance)