efl-mono: Add support for Efl.Class

efl-mono: Add support for Efl.Class

Efl.Class (in practice, the return from the *_class_get() functions) can
be used as argument to functions, like in Efl.Object.provider_find and
Efl.Ui.Widget_Factory.item_class(get/set).

This commits adds support by representing Efl.Class instances
as System.Type in the C# API, allowing someone to do things like:

`factory.ItemClass == typeof(MyFramework.MyButton)`

It also supports user-defined classes that inherit from efl classes.