Retrieves the height of a Actor.
If the actor has a valid allocation, this function will return the height of the allocated area given to the actor.
If the actor does not have a valid allocation, this function will return the actor's natural height, that is the preferred height of the actor.
If you care whether you get the preferred height or the height that has been assigned to the actor, you should probably call a different function like get_allocation_box to retrieve the allocated size or get_preferred_height to retrieve the preferred height.
If an actor has a fixed height, for instance a height that has been assigned using set_height, the height returned will be the same value.
the height of the actor, in pixels