LibGUI: Add on_theme_change callback to Application

This allows an Application without a window to listen for theme changes.

Userland/Libraries/LibGUI/Application.cpp

@@ -311,6 +311,10 @@ void Application::event(Core::Event& event)
                 on_action_leave(action);
         }
     }
+    if (event.type() == GUI::Event::ThemeChange) {
+        if (on_theme_change)
+            on_theme_change();
+    }
     Object::event(event);
 }
 

Userland/Libraries/LibGUI/Application.h

@@ -83,6 +83,7 @@ public:
 
     Function<void(Action&)> on_action_enter;
     Function<void(Action&)> on_action_leave;
+    Function<void()> on_theme_change;
 
     auto const& global_shortcut_actions(Badge<GUI::CommandPalette>) const { return m_global_shortcut_actions; }
 

Userland/Libraries/LibGUI/ConnectionToWindowServer.cpp

@@ -67,6 +67,8 @@ void ConnectionToWindowServer::update_system_theme(Core::AnonymousBuffer const&
     Window::for_each_window({}, [](auto& window) {
         Core::EventLoop::current().post_event(window, make<ThemeChangeEvent>());
     });
+
+    Application::the()->dispatch_event(*make<ThemeChangeEvent>());
 }
 
 void ConnectionToWindowServer::update_system_fonts(String const& default_font_query, String const& fixed_width_font_query)