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/Interfaces/IHasSelectableObject.cs

@@ -0,0 +1,7 @@
+namespace ModernKeePass.Interfaces
+{
+    public interface IHasSelectableObject
+    {
+        ISelectableModel SelectedItem { get; set; }
+    }
+}

ModernKeePass/Interfaces/ISelectableModel.cs

@@ -0,0 +1,7 @@
+namespace ModernKeePass.Interfaces
+{
+    public interface ISelectableModel
+    {
+        bool IsSelected { get; set; }
+    }
+}