LibGUI+LibCore: Move GML property system from LibCore to LibGUI

Since Core::Object properties are really only used by GML now that the
Inspector is long gone, there's no need for these to pollute
Core::Object.

This patch adds a new GUI::Object class to hold properties, and makes
it the new base class of GUI::Window, GUI::Widget and GUI::Layout.
The "instantiate an object by name" mechanism that GML uses is also
hoisted into GUI::Object as well.

Userland/Libraries/LibGUI/Window.cpp

@@ -72,7 +72,7 @@ Window* Window::from_window_id(int window_id)
 }
 
 Window::Window(Core::Object* parent)
-    : Core::Object(parent)
+    : GUI::Object(parent)
     , m_menubar(Menubar::construct())
 {
     if (parent)