Add voice support with availableVoices, voice, and setVoice.

Add signal when voice changes, emit when voice is changed.
In unix backend keep track of which voice names use which locale.
Use locale name for QMultiMap key since QLocale keys aren't matching somehow.
Add slot in example widget to update the locale when localeChanged is received.

Change-Id: I455a7fbdf432d09929b36291f1fb1c673cf24503
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>