Main page now uses Frame to change views when selecting items

Backgrounds unified
Menu items can be disabled thanks to custom ListView control

ModernKeePass/ViewModels/MainVm.cs

@@ -0,0 +1,10 @@
+using System.Collections.ObjectModel;
+using ModernKeePass.Models;
+
+namespace ModernKeePass.ViewModels
+{
+    public class MainVm
+    {
+        public ObservableCollection<MainMenuItem> MainMenuItems { get; set; }
+    }
+}