[core] Fix Index Reset for Tabs in Small Deck Layout (#93)

The index was not reset in the `DeckLayoutSmall` widget, when the user
selected a new deck in the settings widget. This was caused because the
`DefaultTabController` was not rebuild after a new deck was selected, so
that the `initialIndex` value was not used.

This is now fixed by adding a `key` to the `DefaultTabController`, which
corresponds to the selected deck. This means if the user selects a new
deck in the settings the widget will be rebuild and the initial selected
tab will be the first one. If a user selects the same deck or switches
between the small and large layout the tab will be the formerly selected
one.

app/lib/widgets/deck/deck_layout_small.dart

@@ -149,6 +149,7 @@ class DeckLayoutSmall extends StatelessWidget {
     AppRepository app = Provider.of<AppRepository>(context, listen: true);
 
     return DefaultTabController(
+      key: ValueKey(app.activeDeckId),
       initialIndex: _getInitialIndex(context, app.columns.length),
       length: app.columns.length,
       child: Scaffold(