Made AvaloniaPropertyMetadata immutable after property initialization

[MrJul]

What does the pull request do?

This PR makes AvaloniaPropertyMetadata immutable after the AvaloniaProperty using it is initialized.

Rationale:

• It's dangerous to change the metadata after the property has been initialized (no objects currently using the default value would be aware of the change).
• It's dangerous to pass the same metadata to two different properties (the second one might get the overridden defaults from the first without realizing it).
• Having mutable metadata prevents some optimizations, see Improve AvaloniaObject.GetValue performance #15342 (review)

What is the current behavior?

The default property metadata can be modified after a property is initialized, by using the Merge method.

What is the updated/expected behavior with this PR?

An InvalidOperationException is thrown when the metadata is modified.

How was the solution implemented (if it's not obvious)?

A Freeze() method has been added to AvaloniaPropertyMetadata.
It would probably be better to have metadata effectively always immutable and have Merge return a new instance if needed instead of changing the current one, but that ship has sailed for v11 since Merge is public.

Breaking changes

Yes, this is a behavioral breaking change.
In practice, any code that would hit the exception was probably introducing a bug.

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0047313-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0047327-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]