[core] Fix Title Alignment (#28)

When we used the AppBar widget we did not specify the "centerTitle"
property, so that the title was aligned within the default of the
platform. This means on iOS and macOS the title was centered, but on
Android, Windows and Linux the title was aligned on the left side.

This wasn't intended and we want to have the same style on all
Platforms, so that the title is now centered on all Platforms like it
was already done on iOS and macOS.

app/lib/main.dart

@@ -149,6 +149,7 @@ class FeedDeckApp extends StatelessWidget {
             ),
             canvasColor: Constants.canvasColor,
             appBarTheme: const AppBarTheme(
+              centerTitle: true,
               backgroundColor: Constants.appBarBackgroundColor,
               scrolledUnderElevation: Constants.scrolledUnderElevation,
               elevation: Constants.appBarElevation,