Remove appbase as an external dependency.

lib/appbase/common/Singleton.hpp

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <boost/noncopyable.hpp>
+
+namespace AB_NAMESPACE {
+
+class Settings;
+class Paths;
+
+class Singleton : boost::noncopyable
+{
+public:
+    virtual ~Singleton() = default;
+
+    virtual void initialize(Settings &settings, Paths &paths)
+    {
+        (void)(settings);
+        (void)(paths);
+    }
+
+    virtual void save()
+    {
+    }
+};
+
+}  // namespace AB_NAMESPACE