feat: implement Default for EchidnaSidebar

EchidnaSidebar doesn't take any parameters, thus allowing Default to be easily implemented on it.

src/components/sidebar/mod.rs

@@ -15,3 +15,9 @@ impl EchidnaSidebar {
         glib::Object::new(&[]).expect("Failed to create 'EchidnaSidebar' component.")
     }
 }
+
+impl Default for EchidnaSidebar {
+    fn default() -> Self {
+        Self::new()
+    }
+}