change open_screen return a screen

[toshiwo]

hi, change branch version-1.0 ( #178 )

I came up with this idea when I was using modal view.

before:

def action_open_modal
  @modal_screen = ModalScreen.new
  open @modal_screen, moda: true
end

after:

def action_open_modal
  @modal_screen = open ModalScreen, moda: true
end