Main page now uses Frame to change views when selecting items

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

ModernKeePass/Interfaces/IIsEnabled.cs

@@ -0,0 +1,7 @@
+namespace ModernKeePass.Interfaces
+{
+    public interface IIsEnabled
+    {
+        bool IsEnabled { get; }
+    }
+}