ThemeEditor: Disable focus for demo widgets

Pressing tab would focus on the demo widgets instead of switching
between the combo box and color input field. Instead set the focus
policy to NoFocus for all the widgets.

Userland/Applications/ThemeEditor/PreviewWidget.cpp

@@ -49,6 +49,11 @@ private:
         m_statusbar->set_text("Status bar");
         m_editor = add<GUI::TextEditor>();
         m_editor->set_text("Text editor\nwith multiple\nlines.");
+
+        for_each_child_widget([](auto& child) {
+            child.set_focus_policy(GUI::FocusPolicy::NoFocus);
+            return IterationDecision::Continue;
+        });
     }
 
     virtual void resize_event(GUI::ResizeEvent&) override