Add missing i18n to native macOS receiver selector and refactor

app/NativeMacReceiverSelector/models/InitData.swift

@@ -0,0 +1,12 @@
+struct InitData : Codable {
+    let receivers: [Receiver]
+    let defaultMediaType: MediaType
+
+    let i18n_extensionName: String
+    let i18n_castButtonTitle: String
+    let i18n_mediaTypeApp: String
+    let i18n_mediaTypeTab: String
+    let i18n_mediaTypeScreen: String
+    let i18n_mediaSelectCastLabel: String
+    let i18n_mediaSelectToLabel: String
+}

app/NativeMacReceiverSelector/models/MediaType.swift

@@ -0,0 +1,3 @@
+enum MediaType : Int, Codable {
+    case app, tab, screen
+}

app/NativeMacReceiverSelector/models/Receiver.swift

@@ -0,0 +1,6 @@
+struct Receiver : Codable {
+    let friendlyName: String
+    let host: String
+    let id: String
+    let port: Int
+}

app/NativeMacReceiverSelector/models/ReceiverSelection.swift

@@ -0,0 +1,4 @@
+struct ReceiverSelection : Codable {
+    let receiver: Receiver
+    let mediaType: MediaType
+}