Implement SetItems method in ResourceDictionary

[StefanKoell]

What does the pull request do?

This PR provides a new method to the ResourceDictionary called AddOrUpdateRange which allows to batch multiple updates in one go and notify all subscribers about the change at the very end once the resources have been updated.

See discussion: #18346

What is the current behavior?

At the moment, adding or updating existing resources are immediately notifying all subscribers about resource changes. In complex application this can become quite expensive and slow when your updating multiple (tens or hundreds) resources by code.

What is the updated/expected behavior with this PR?

When using the AddOrUpdateRange methods, the notification of all subscribers happens only once at the end of the method, regardless of the number of updates performed.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None.

Obsoletions / Deprecations

None.

Fixed issues

#18346

[MrJul]

Public API for review:

namespace Avalonia.Controls
{
    public class ResourceDictionary : ResourceProvider, IResourceDictionary, IThemeVariantProvider
    {
+        public void AddOrUpdateRange(IEnumerable<KeyValuePair<object, object?>> values);
    }
}

[maxkatz6]

This API should be added to the AvaloniaDictionary as well, I would say. Even as a single Reset operation, it still would be better.

[MrJul]

The API review focused on the naming, as we're fine with the behavior.

Points:

• ConcurrentDictionary.AddOrUpdate exists but is a bit different there, as it takes a function to update the existing item.
• ImmutableArray.SetItems has the exact same API shape as this PR.

After some debate, SetItems won.

[MrJul]

LGTM, thank you!