undo

$canvaseditor undo: makes last user-action undone.

obj method canvaseditor undo {} {
  # Letzter Befehl retour
  if {[my undonePtr] < [llength [my undoStack]]} then {
    set code [lindex [my undoStack] end-[my undonePtr]]
    my undonePtr [+ [my undonePtr] 1]
    set before [$self elements]
    # den Code als Lambda-Körper ausführen
    apply [list "" $code]
    $self statusLine Undo
  } else {
    $self statusLine No more undo!
  }
}