LibCore: Make CObject reference-counted

Okay, I've spent a whole day on this now, and it finally kinda works!
With this patch, CObject and all of its derived classes are reference
counted instead of tree-owned.

The previous, Qt-like model was nice and familiar, but ultimately also
outdated and difficult to reason about.

CObject-derived types should now be stored in RefPtr/NonnullRefPtr and
each class can be constructed using the forwarding construct() helper:

    auto widget = GWidget::construct(parent_widget);

Note that construct() simply forwards all arguments to an existing
constructor. It is inserted into each class by the C_OBJECT macro,
see CObject.h to understand how that works.

CObject::delete_later() disappears in this patch, as there is no longer
a single logical owner of a CObject.

Applications/Taskbar/TaskbarWindow.cpp

@@ -46,7 +46,7 @@ void TaskbarWindow::on_screen_rect_change(const Rect& rect)
     set_rect(new_rect);
 }
 
-GButton* TaskbarWindow::create_button(const WindowIdentifier& identifier)
+NonnullRefPtr<GButton> TaskbarWindow::create_button(const WindowIdentifier& identifier)
 {
     auto button = TaskbarButton::construct(identifier, main_widget());
     button->set_size_policy(SizePolicy::Fixed, SizePolicy::Fixed);