Change Skin -> Theme

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

Oqtane.Client/Themes/Theme3/HorizontalLayout.razor

@@ -0,0 +1,17 @@
+@using Oqtane.Themes
+@using Oqtane.Client.Shared
+@inherits ThemeBase
+
+<div class="row px-4">
+    <div class="col-sm">
+        <Pane Name="Left" />
+    </div>
+    <div class="col-sm">
+        <Pane Name="Right" />
+    </div>
+</div>
+
+@functions {
+    public override string Name { get { return "Horizontal Layout"; } }
+    public override string Panes { get { return "Left;Right"; } }
+}