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/AboutPage.xaml.cs

@@ -0,0 +1,15 @@
+// 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 AboutPage
+    {
+        public AboutPage()
+        {
+            InitializeComponent();
+        }
+    }
+}