FloatableElement: Add config for 'hideWhenOutOfView'
FloatableElement: Add config for 'hideWhenOutOfView'

Allow floatable elements to not automatically be hidden when their
containers are out of view.

Change-Id: I92b794d5556302099df72af36620299536bf6fe4

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui