Replace most inclusions of MemoryX.h with <memory> ...

... Most often it was needed for a custom definition of std::make_unique, but
we build C++14 now.

src/Prefs.cpp

@@ -61,6 +61,7 @@
 
 #include "Internat.h"
 #include "MemoryX.h"
+#include <memory>
 
 std::unique_ptr<FileConfig> ugPrefs {};