Layouts: Add support for setting a default layout

This is a better approach than setting a ThemeSetting Default, since Genesis then sets the UI to highlight the default layout correctly.

src/Layouts.php

@@ -26,6 +26,7 @@
  *         Layouts::SIDEBAR_CONTENT_SIDEBAR,
  *         Layouts::SIDEBAR_SIDEBAR_CONTENT,
  *     ],
+ *     Layouts::DEFAULTLAYOUT => __genesis_return_full_width_content()
  * ];
  * ```
  *
@@ -47,6 +48,7 @@ class Layouts extends Brick
 {
     const REGISTER   = 'register';
     const UNREGISTER = 'unregister';
+    const DEFAULTLAYOUT = 'default-layout';
 
     const FULL_WIDTH_CONTENT = 'full-width-content';
     const CONTENT_SIDEBAR = 'content-sidebar';
@@ -69,6 +71,10 @@ public function apply()
             $unregisterConfig = $this->config->getSubConfig(self::UNREGISTER);
             $this->unregister($unregisterConfig->getArrayCopy());
         }
+
+        if ($this->config->hasKey(self::DEFAULTLAYOUT)) {
+            $this->setDefault($this->config->getKey(self::DEFAULTLAYOUT));
+        }
     }
 
     /**
@@ -94,4 +100,16 @@ protected function unregister(array $args)
             \genesis_unregister_layout($value);
         });
     }
+
+    /**
+     * Set a default Genesis layout.
+     *
+     * Allow a user to identify a layout as being the default layout on a new install, as well as serve as the fallback layout.
+     *
+     * @param string $layout Layout handle.
+     */
+    protected function setDefault(string $layout)
+    {
+        \genesis_set_default_layout($layout);
+    }
 }