LibGUI: More work on GCheckBox.

- Make it track the mouse cursor just like GButton does so that changes only
  get committed if the mouseup event happens while inside the widget rect.

- Draw a focus rect around the box when appropriate.

- When focused, support toggling the checked state with the space bar.

Userland/guitest2.cpp

@@ -14,6 +14,7 @@
 #include <LibGUI/GButton.h>
 #include <LibGUI/GEventLoop.h>
 #include <LibGUI/GTextBox.h>
+#include <LibGUI/GCheckBox.h>
 
 static GWindow* make_font_test_window();
 static GWindow* make_launcher_window();
@@ -117,6 +118,10 @@ GWindow* make_launcher_window()
     auto* other_textbox = new GTextBox(widget);
     other_textbox->set_relative_rect({ 5, 140, 90, 20 });
 
+    auto* checkbox = new GCheckBox(widget);
+    checkbox->set_relative_rect({ 5, 170, 90, 20 });
+    checkbox->set_caption("CheckBox");
+
     window->set_focused_widget(textbox);
 
     return window;