Option to serialize members of base types first

[n9]

Is your feature request related to a problem? Please describe.
Members are serialized in the order returned by reflection, with members of base types coming last, which is sometimes not desirable.

Describe the solution you'd like
Remove sealed from ReadablePropertiesTypeInspector and and from ReflectionPropertyDescriptor.

YamlDotNet/YamlDotNet/Serialization/TypeInspectors/ReadablePropertiesTypeInspector.cs

Line 33 in 485daaa

public sealed class ReadablePropertiesTypeInspector : ReflectionTypeInspector

YamlDotNet/YamlDotNet/Serialization/TypeInspectors/ReadablePropertiesTypeInspector.cs

Line 63 in 485daaa

private sealed class ReflectionPropertyDescriptor : IPropertyDescriptor

And extract ReflectionPropertyDescriptor to a top-level class (see Additional context), or make protected.

Describe alternatives you've considered
Copy/paste ReadablePropertiesTypeInspector and tweak the order.

Additional context
WritablePropertiesTypeInspector can be also later refactored to share code with ReadablePropertiesTypeInspector.

YamlDotNet/YamlDotNet/Serialization/TypeInspectors/WritablePropertiesTypeInspector.cs

Line 33 in 485daaa

public sealed class WritablePropertiesTypeInspector : ReflectionTypeInspector

[EdwardCooke]

To keep this change simple I'll just remove sealed and make the type protected for now.

[n9]

@EdwardCooke Thank you!

I realized that I still need to copy part of code to achieve ordering (as some members are private). I try to address this by PR: #987