This layer's window title.
If the frontmost layer does not have visible history,
the browser window will show the title of an ancestor layer.
This property will return the title the layer would use if it had visible history.
If this layer does not affect browser history, this property will
still return the title the layer would otherwise use.
When this layer opens a child layer with visible history, the browser window will change to the child
layer's title. When the child layer is closed, this layer's title will be restored.
