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/LibWeb/Loader/ResourceLoader.cpp

@@ -205,11 +205,4 @@ bool ResourceLoader::is_port_blocked(int port)
     return false;
 }
 
-void ResourceLoader::save_to(JsonObject& object)
-{
-    Object::save_to(object);
-    object.set("pending_loads", m_pending_loads);
-    object.set("user_agent", m_user_agent);
-}
-
 }