LibCore: Make Core::Object properties more dynamic

Instead of everyone overriding save_to() and set_property() and doing
a pretty asymmetric job of implementing the various properties, let's
add a bit of structure here.

Object properties are now represented by a Core::Property. Properties
are registered with a getter and setter (optional) in constructors.
I've added some convenience macros for creating and registering
properties, but this does still feel a bit bulky. We'll have to
iterate on this and see where it goes.

Libraries/LibGUI/BoxLayout.h

@@ -44,8 +44,6 @@ public:
 protected:
     explicit BoxLayout(Gfx::Orientation);
 
-    virtual void save_to(JsonObject &) override;
-
 private:
     Gfx::Orientation m_orientation;
 };