Use GirCore subclasses

[badcel]

WIP and discussion

• This is a sample on how the current conversion to a GirCore would look like if there is no paramterless constructor anymore.
• Please ignore the compiler errors: This is build for Manually set instance cache during object creation gircore/gir.core#1437
• Readonly member / properties support is lost
• Initialize method can be used for non parametrized initialization. This would even work if the instance is created in C and not known to dotnet
• The New factory method is used to configure the instance in a certain way

Insights:

• All subclasses must be touched, there is no soft migration
• Certain dotnet compiler optimizations might be lost

Questions:

• Do we need support to install properties / construct properties inside GObject to (eventually) support read only properties? Should this be delivered later or is it a prerequisite? Partial properties are supported with dotnet 9 and could be handy for this case. As GirCore supports dotnet 8 this is available with dotnet 9. If this works NewWithProperties could be used to set those properties.
• Do we need a soft migration? This would lead to more in complete / seemingly buggy composite template support because eventually templates would not be initialized correctly if the instances are not known to C# (in case of hierarchical composite templates).

[cameronwhite]

I've added bug #2050 for us to upgrade to the new gir.core preview, so I think we can close this PR. Thanks for all the great work on the new release! 🎉