[RP] save/restore frame-set

[Stefan Sandstrom]

An idéa:
If there was a general mechanism to save and restore the frames currently
on the screen many useful things could be done, e.g.:

  "un-only": could be done by saving the current "frame-state", do an only
             and then restore the saved state.
            
   virtual screens: It's just a vector of "frame-states".
   
   undo: A general "frame-op-undo" by saving the state before each op for
         the last N ops.

-stefan