Add rudimentary navigation

Gemini/ContentView.swift

@@ -17,7 +17,22 @@ struct ContentView: View {
     @State var errorMessage: String?
     
     var body: some View {
-        mainView
+        VStack {
+            HStack {
+                Button(action: navigator.back) {
+                    Image(systemName: "chevron.left")
+                }.disabled(navigator.backStack.isEmpty)
+                Button(action: navigator.forward) {
+                    Image(systemName: "chevron.right")
+                }.disabled(navigator.forwardStack.isEmpty)
+                TextField("URL", text: Binding<String>(get: {
+                    self.navigator.currentURL.absoluteString
+                }, set: { (newValue) in
+                    self.navigator.currentURL = URL(string: newValue)!
+                }))
+            }.padding([.leading, .trailing])
+            mainView.frame(maxWidth: .infinity, maxHeight: .infinity)
+        }
             .frame(minWidth: 480, maxWidth: .infinity, minHeight: 300, maxHeight: .infinity)
             .onReceive(navigator.$currentURL, perform: self.urlChanged)
     }

Gemini/NavigationManager.swift

@@ -10,13 +10,29 @@ import Foundation
 class NavigationManager: ObservableObject {
     
     @Published var currentURL: URL
+    @Published var backStack = [URL]()
+    @Published var forwardStack = [URL]()
     
     init(url: URL) {
         self.currentURL = url
     }
     
     func changeURL(_ url: URL) {
-        self.currentURL = url
+        backStack.append(currentURL)
+        currentURL = url
+        forwardStack = []
+    }
+    
+    func back() {
+        guard !backStack.isEmpty else { return }
+        forwardStack.insert(currentURL, at: 0)
+        currentURL = backStack.removeLast()
+    }
+    
+    func forward() {
+        guard !forwardStack.isEmpty else { return }
+        backStack.append(currentURL)
+        currentURL = forwardStack.removeFirst()
     }
     
 }