Major changes in MainPage: now is layout dependant (with a base, to prepare for Settings)

Main sub pages also changed to better work with Main (but needs improvement)
Added a Settings Page stub

ModernKeePass/Pages/MainPageFrames/OpenDatabasePage.xaml.cs

@@ -0,0 +1,49 @@
+using System;
+using Windows.Storage.Pickers;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+using Windows.UI.Xaml.Navigation;
+using ModernKeePass.Events;
+using ModernKeePass.ViewModels;
+
+// The Blank Page item template is documented at http://go.microsoft.com/fwlink/?LinkId=234238
+
+namespace ModernKeePass.Pages
+{
+    /// <summary>
+    /// An empty page that can be used on its own or navigated to within a Frame.
+    /// </summary>
+    public sealed partial class OpenDatabasePage
+    {
+        private Frame _mainFrame;
+
+        public OpenVm Model => (OpenVm)DataContext;
+
+        public OpenDatabasePage()
+        {
+            InitializeComponent();
+        }
+
+        protected override void OnNavigatedTo(NavigationEventArgs e)
+        {
+            base.OnNavigatedTo(e);
+            _mainFrame = e.Parameter as Frame;
+        }
+
+        private async void ButtonBase_OnClick(object sender, RoutedEventArgs e)
+        {
+            var picker =
+                new FileOpenPicker
+                {
+                    ViewMode = PickerViewMode.List,
+                    SuggestedStartLocation = PickerLocationId.DocumentsLibrary
+                };
+            picker.FileTypeFilter.Add(".kdbx");
+            
+            // Application now has read/write access to the picked file
+            var file = await picker.PickSingleFileAsync();
+            if (file == null) return;
+            Model.OpenFile(file);
+        }
+    }
+}