code: func Center(width, height int, p tview.Primitive) tview.Primitive {
  	return tview.NewFlex().
  		AddItem(tview.NewBox(), 0, 1, false).
  		AddItem(tview.NewFlex().
  			SetDirection(tview.FlexRow).
  			AddItem(tview.NewBox(), 0, 1, false).
  			AddItem(p, height, 1, true).
  			AddItem(tview.NewBox(), 0, 1, false), width, 1, true).
  		AddItem(tview.NewBox(), 0, 1, false)
  }
docs: // Center returns a new primitive which shows the provided primitive in its
  // center, given the provided primitive's size.