LablTk: create new windows

Generate new windows daughters of the original top level. Note that initially the button "Destroy" is in state `Disabled, i.e. it cannot be pressed.

Pressing "Create" makes a new window, and causes the "Destroy" button to be in state`Normal, so that it can react to the action of the user, while the "Create" button becomes disabled.

Caveat: if I put an Entry widget in the daughter window, I cannot manage to retrieve its contents until the mother window gets closed, which can be annoying.