Qt: Add box icon to line box debug menu action

(cherry picked from commit 3e7faae6472d9696637d58f30994199a28dddc01)

Ladybird/Qt/BrowserWindow.cpp

@@ -437,6 +437,7 @@ BrowserWindow::BrowserWindow(Vector<URL::URL> const& initial_urls, WebView::Cook
 
     m_show_line_box_borders_action = new QAction("Show Line Box Borders", this);
     m_show_line_box_borders_action->setCheckable(true);
+    m_show_line_box_borders_action->setIcon(load_icon_from_uri("resource://icons/16x16/box.png"sv));
     debug_menu->addAction(m_show_line_box_borders_action);
     QObject::connect(m_show_line_box_borders_action, &QAction::triggered, this, [this] {
         bool state = m_show_line_box_borders_action->isChecked();

Ladybird/cmake/ResourceFiles.cmake

@@ -19,6 +19,7 @@ list(TRANSFORM FONTS PREPEND "${SERENITY_SOURCE_DIR}/Base/res/fonts/")
 set(16x16_ICONS
     app-browser.png
     app-system-monitor.png
+    box.png
     audio-volume-high.png
     audio-volume-muted.png
     close-tab.png