settings: use default audio devices

Refactors audio settings to avoid list model code duplication. This code
could be greatly simplified by using string lists, however the combo-box
component design is highly dependent on QAbstractItemModel based models.

Also translates the handlebarred strings upon presentation, which is
currently used to translate the "Default" prefix for the first device
item.

Gitlab: #346
Change-Id: I5ed282d29cc4ec6a090a9cdf47b0459f0db2a99b