Change Skin -> Theme

To better align with commonly used terminology in industry renamed all references from Skin -> Theme.

Oqtane.Client/Themes/ITheme.cs

@@ -0,0 +1,15 @@
+using System;
+
+namespace Oqtane.Themes
+{
+    public interface ITheme
+    {
+        string Name { get; }
+        string Version { get; }
+        string Owner { get; }
+        string Url { get; }
+        string Contact { get; }
+        string License { get; }
+        string Dependencies { get; }
+    }
+}