Fix sound control used in multistring slice

The LabelMultiStringView class had to be refactored to get sound widgets
to work properly in multilingual text slices (actually to get the text
portions to work properly when a sound control is present).  See FWNX-1158
for details.

Authored with Steve Mc.

Change-Id: Ifb0178faaa7e25ea45b1e8a938fbfce91caac4f2