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/CMakeLists.txt

@@ -79,6 +79,7 @@ set(SOURCES
     MouseTracker.cpp
     MultiView.cpp
     Notification.cpp
+    Object.cpp
     OpacitySlider.cpp
     Painter.cpp
     PasswordInputDialog.cpp
@@ -87,6 +88,7 @@ set(SOURCES
     Process.cpp
     ProcessChooser.cpp
     Progressbar.cpp
+    Property.cpp
     RadioButton.cpp
     RangeSlider.cpp
     RegularEditingEngine.cpp