app: Use Adw.StyleManager to enable dark-theme

Latest libadwaita (for reasons unknown to me) totally ignores/breaks dark theme usage with gtk_application_prefer_dark_theme property. Lets just try using the new Adw.StyleManager without asking questions why.

src/appBase.js

@@ -96,13 +96,6 @@ class ClapperAppBase extends Gtk.Application
             if(accels)
                 this.set_accels_for_action(`app.${name}`, accels);
         }
-
-        const gtkSettings = Gtk.Settings.get_default();
-        settings.bind(
-            'dark-theme', gtkSettings,
-            'gtk-application-prefer-dark-theme',
-            Gio.SettingsBindFlags.GET
-        );
         this.doneFirstActivate = true;
     }
 });