Adds Mac Preferences

This makes use of `@AppStorage` for preferences. Severeral more need to migrated from AppDefaults etc.

Multiplatform/macOS/Preferences/Views/GeneralPreferencesView.swift

@@ -0,0 +1,33 @@
+//
+//  GeneralPreferencesView.swift
+//  macOS
+//
+//  Created by Stuart Breckenridge on 27/6/20.
+//
+
+import SwiftUI
+
+struct GeneralPreferencesView: View {
+    
+    @ObservedObject private var preferences = MacPreferences()
+    
+    var body: some View {
+        VStack {
+            Form {
+                Picker("Refresh Feeds",
+                       selection: $preferences.refreshFrequency,
+                       content: {
+                    ForEach(0..<preferences.refreshIntervals.count, content: {
+                        Text(preferences.refreshIntervals[$0])
+                    })
+                }).frame(width: 300, alignment: .center)
+                
+                Toggle("Open webpages in background in browser", isOn: $preferences.openInBackground)
+                
+                Toggle("Show Unread Count in Dock", isOn: $preferences.showUnreadCountInDock)
+            }
+            Spacer()
+        }.frame(width: 300, alignment: .center)
+    }
+    
+}