LibGUI: Add a way to highlight the focused widget for debugging

You can now pass --gui-focus-debug to any GUI::Application and it will
draw a cyan rectangle around the currently focused widget.

Libraries/LibGUI/Application.cpp

@@ -56,8 +56,14 @@ Application::Application(int argc, char** argv)
     WindowServerConnection::the();
     if (argc > 0)
         m_invoked_as = argv[0];
-    for (int i = 1; i < argc; i++)
+    for (int i = 1; i < argc; i++) {
-        m_args.append(argv[i]);
+        String arg(argv[i]);
+
+        if (arg == "--gui-focus-debug")
+            m_focus_debugging_enabled = true;
+
+        m_args.append(move(arg));
+    }
 }
 
 Application::~Application()

Libraries/LibGUI/Application.h

@@ -68,6 +68,8 @@ public:
 
     void set_system_palette(SharedBuffer&);
 
+    bool focus_debugging_enabled() const { return m_focus_debugging_enabled; }
+
 private:
     OwnPtr<Core::EventLoop> m_event_loop;
     RefPtr<MenuBar> m_menubar;
@@ -77,6 +79,7 @@ private:
     class TooltipWindow;
     RefPtr<TooltipWindow> m_tooltip_window;
     bool m_quit_when_last_window_deleted { true };
+    bool m_focus_debugging_enabled { false };
     String m_invoked_as;
     Vector<String> m_args;
 };

Libraries/LibGUI/Widget.cpp

@@ -242,6 +242,13 @@ void Widget::handle_paint_event(PaintEvent& event)
         Painter painter(*this);
         painter.draw_rect(rect(), Color::Magenta);
     }
+
+    if (Application::the().focus_debugging_enabled()) {
+        if (is_focused()) {
+            Painter painter(*this);
+            painter.draw_rect(rect(), Color::Cyan);
+        }
+    }
 }
 
 void Widget::set_layout(NonnullRefPtr<Layout> layout)