Avoid Reflection

[badcel]

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Fixes: #397, #919

Before working on this there should probably be a PR which introduces a new way of registering classes and subclasses. Additionally there should be a source generator which uses this way to register the types and subclasses with the existing TypeDictionary. Subclasses can be registered in their static constructor as they can't be created by GObject before they were used in c#. If this is done there can be work done to remove reflection and the TypeDictionary. In this PR.

Todo:

• Ensure that classes and subclass can be created with their own gtype.
• Allow users to Register their subclasses manually so all types are known to the typesystem
• Create SourceGenerator to automatically register all subclasses (optional)
• Find a way to remove the TypeDictionary and pass on the GType through constructor chain if a subclass is instantiated manually thus avoiding the lookup in the type dictionary
• Make sure that it is possible to get an instance of a subclass even if the metod returns a GObject.Object