Facade class for basic UI, injection of a wxWidgets implementation...

... but none of the methods is defined yet.

The intention is to inject dependencies on wxWidgets (or other) toolkit so that
lower-level files have less build dependency on wxCore classes or on the
event loop.

libraries/lib-basic-ui/BasicUI.cpp

@@ -0,0 +1,25 @@
+/*!********************************************************************
+
+Audacity: A Digital Audio Editor
+
+@file BasicUI.cpp
+
+Paul Licameli
+
+**********************************************************************/
+#include "BasicUI.h"
+
+namespace  BasicUI {
+Services::~Services() = default;
+
+static Services *theInstance = nullptr;
+
+Services *Get() { return theInstance; }
+
+Services *Install(Services *pInstance)
+{
+   auto result = theInstance;
+   theInstance = pInstance;
+   return result;
+}
+}