Enable back button and introduce behavior

Signed-off-by: Danilo Aimini <82231674+AMZN-daimini@users.noreply.github.com>

Code/Framework/AzToolsFramework/AzToolsFramework/Prefab/PrefabFocusHandler.cpp

@@ -127,6 +127,11 @@ namespace AzToolsFramework::Prefab
         return m_instanceFocusPath;
     }
 
+    const int PrefabFocusHandler::GetPrefabFocusPathLength()
+    {
+        return m_instanceFocusVector.size();
+    }
+
     void PrefabFocusHandler::OnEntityStreamLoadSuccess()
     {
         // Focus on the root prefab (AZ::EntityId() will default to it)