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/imageareatoollistener.h

@@ -28,20 +28,20 @@ class ImageAreaToolListener
 
 public:
     virtual ~ImageAreaToolListener() {}
-    virtual void spotWBselected (int x, int y, Thumbnail* thm = NULL) {}
+    virtual void spotWBselected (int x, int y, Thumbnail* thm = nullptr) {}
     virtual int  getSpotWBRectSize ()
     {
         return 8;
     }
     virtual void cropSelectionReady () {}
-    virtual void rotateSelectionReady (double rotate_deg, Thumbnail* thm = NULL) {}
+    virtual void rotateSelectionReady (double rotate_deg, Thumbnail* thm = nullptr) {}
     virtual ToolBar* getToolBar ()
     {
-        return NULL;
+        return nullptr;
     }
-    virtual CropGUIListener* startCropEditing (Thumbnail* thm = NULL)
+    virtual CropGUIListener* startCropEditing (Thumbnail* thm = nullptr)
     {
-        return NULL;
+        return nullptr;
     }
 };