window: focus content area instead of sidebar on startup

Scroll events were going to the sidebar list because GTK's default
focus traversal landed on the article_list_view.  Call grab_focus()
on the WebView at the end of constructed() so the content area
receives input by default.

src/window.rs

@@ -137,6 +137,7 @@ pub mod imp {
             self.setup_capture_keys();
             self.restore_from_cache();
             self.auto_login();
+            self.web_view.grab_focus();
         }
     }