Help: Set tree view as focused widget

This allows you to use the start using the keys to navigate the
menu immediately, instead of having to click on the tree view first.

Applications/Help/main.cpp

@@ -158,6 +158,7 @@ int main(int argc, char* argv[])
     app.set_menubar(move(menubar));
 
     window->set_main_widget(widget);
+    window->set_focused_widget(tree_view);
     window->show();
 
     window->set_icon(load_png("/res/icons/16x16/book.png"));