brighten

$group brighten outline (factor): changes line colors of if not constraint
$group brighten fill (factor): changes fill colors of if not constraint

obj method group brighten {what factor} {
  switch -exact -- $what {
    outline {
      if {![$self constraint linecolor]} then {
        $self elements brighten $what $factor
      }
    }
    fill {
      if {![$self constraint fillcolor]} then {
        $self elements brighten $what $factor
      }
    }
  }
}