Initial implementation of UI automation.

Follows WPF/UWP API as closely as possible. Limited to win32 right now. Broken in many places.

src/Avalonia.Controls/Automation/AutomationElementIdentifiers.cs

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+using Avalonia.Automation.Peers;
+
+namespace Avalonia.Automation
+{
+    /// <summary>
+    /// Contains values used as automation property identifiers by UI Automation providers.
+    /// </summary>
+    public static class AutomationElementIdentifiers
+    {
+        /// <summary>
+        /// Identifies the bounding rectangle automation property. The bounding rectangle property
+        /// value is returned by the <see cref="AutomationPeer.GetBoundingRectangle"/> method.
+        /// </summary>
+        public static AutomationProperty BoundingRectangleProperty { get; } = new();
+
+        /// <summary>
+        /// Identifies the class name automation property. The class name property value is returned
+        /// by the <see cref="AutomationPeer.GetClassName"/> method.
+        /// </summary>
+        public static AutomationProperty ClassNameProperty { get; } = new();
+
+        /// <summary>
+        /// Identifies the name automation property. The class name property value is returned
+        /// by the <see cref="AutomationPeer.GetName"/> method.
+        /// </summary>
+        public static AutomationProperty NameProperty { get; } = new();
+    }
+}

src/Avalonia.Controls/Automation/AutomationLiveSetting.cs

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+namespace Avalonia.Automation
+{
+    /// <summary>
+    /// Describes the notification characteristics of a particular live region
+    /// </summary>
+    public enum AutomationLiveSetting
+    {
+        /// <summary>
+        /// The element does not send notifications if the content of the live region has changed.
+        /// </summary>
+        Off = 0,
+
+        /// <summary>
+        /// The element sends non-interruptive notifications if the content of the live region has
+        /// changed. With this setting, UI Automation clients and assistive technologies are expected 
+        /// to not interrupt the user to inform of changes to the live region.
+        /// </summary>
+        Polite = 1,
+
+        /// <summary>
+        /// The element sends interruptive notifications if the content of the live region has changed. 
+        /// With this setting, UI Automation clients and assistive technologies are expected to interrupt 
+        /// the user to inform of changes to the live region.
+        /// </summary>
+        Assertive = 2,
+    }
+}
+