Rearrange voice selection handling

Currently, we return many voices for the Icelandic locale, "abducting" the locale property `variant` for the voice name. The user has to explictely choose the voice by selecting the locale, instead of choosing it via the voice settings.

We should rather make the voice choosable inside the app and only return the selected voice for the "favorite" voice of is_IS. If the user presses the voice settings inside the TTS system settings, we should refer him to these settings. We should only add more voices for the locale, if we really support different locales.