Appearance Customization: add Appearance protocol

Add the `Appearance` protocol definition.

Author: compnerd

Sources/SwiftWin32/Appearance Customization/Appearance.swift

@@ -0,0 +1,30 @@
+/**
+ * Copyright © 2020 Saleem Abdulrasool <compnerd@compnerd.org>
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ **/
+
+/// A collection of methods that gives you access to the appearance proxy for a
+/// class.
+public protocol Appearance {
+  // MARK - Appearance Methods
+
+  /// Returns the appearance proxy for the receiver.
+  static func appearance() -> Self
+
+  /// Returns the appearance proxy for the receiver that has the passed trait
+  /// collection.
+  static func appearance(for trait: TraitCollection) -> Self
+
+  /// Returns the appearance proxy for the object when it is contained in the
+  /// hierarchy the specified classes describe.
+  static func appearance(whenContainedInInstancesOf containerTypes: [AppearanceContainer.Type])
+      -> Self
+
+  /// Returns the appearance proxy for the object when it is contained in the
+  /// hierarchy the specified classes describe and has the specified trait
+  /// collection.
+  static func appearance(for trait: TraitCollection,
+                         whenContainedInInstancesOf containerTypes: [AppearanceContainer.Type]) -> Self
+}

Sources/SwiftWin32/Appearance Customization/AppearanceContainer.swift

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+/**
+ * Copyright © 2020 Saleem Abdulrasool <compnerd@compnerd.org>
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ **/
+
+/// A protocol that a class must adopt to allow appearance customization using
+/// the `Appearance` API.
+public protocol AppearanceContainer {
+}