demos: Add 'invisibleLabel' to quiet ButtonMenuSelectWidget
demos: Add 'invisibleLabel' to quiet ButtonMenuSelectWidget

Adding label for accessible demo

Change-Id: I7a9e5a55dc12a30c7ee5aa12eada0fd1bb612312

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