Main page now uses Frame to change views when selecting items

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

ModernKeePass/Models/MainMenuItem.cs

@@ -0,0 +1,17 @@
+using System;
+using ModernKeePass.Interfaces;
+
+namespace ModernKeePass.Models
+{
+    public class MainMenuItem: IIsEnabled
+    {
+        public string Title { get; set; }
+        public Type PageType { get; set; }
+        public bool IsEnabled => PageType != null;
+
+        public override string ToString()
+        {
+            return Title;
+        }
+    }
+}