Only use specific std:: items we need.

Also, fix `using std::unique_ptr` to `using std::make_unique` in
stdshims.h

src/core/ComboBoxModel.cpp

@@ -25,7 +25,8 @@
 #include "ComboBoxModel.h"
 #include "embed.h"
 
-using namespace std;
+using std::unique_ptr;
+using std::move;
 
 void ComboBoxModel::addItem( QString item, unique_ptr<PixmapLoader> loader )
 {

src/gui/editors/PianoRoll.cpp

@@ -67,7 +67,7 @@
 #define MiddleButton MidButton
 #endif
 
-using namespace std;
+using std::move;
 
 typedef AutomationPattern::timeMap timeMap;