Apply modernize-use-nullptr

Setup:
- `mkdir tidy; cd tidy`
- `cmake .. -DCMAKE_BUILD_TYPE=debug -DPROC_TARGET_NUMBER=1 -DCACHE_NAME_SUFFIX=4 -DBINDIR=. -DDATADIR=. -DBUILD_BUNDLE=ON -DWITH_LTO=OFF -DOPTION_OMP=OFF -DCMAKE_EXPORT_COMPILE_COMMANDS=ON`
- `cd ..`
- `find -name '*.cc' -exec clang-tidy-3.8 -header-filter=.* -p=tidy -fix-errors -checks=modernize-use-nullptr {} \;`

rtgui/curveeditorgroup.h

@@ -85,7 +85,7 @@ public:
         return displayedCurve;
     }
     //void on_realize ();
-    CurveEditor* addCurve(CurveType cType, Glib::ustring curveLabel, Gtk::Widget *relatedWidget = NULL, bool periodic = true);
+    CurveEditor* addCurve(CurveType cType, Glib::ustring curveLabel, Gtk::Widget *relatedWidget = nullptr, bool periodic = true);
 
 protected:
     //void curveTypeToggled ();