backend/clx/mirror: Initial support for non-GRAFT mirrored sheets.

  * This is just the method for REALIZE-MIRROR at this point, no
hookups for event handling (which we don't have any code for yet),
no support for DESTROY-MIRROR (seven years bad luck), none of the
special magical integration for things like (SETF SHEET-ENABLED),
but enough that we can use it to put a window on the screen.