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/imageareapanel.cc

@@ -18,7 +18,7 @@
  */
 #include "imageareapanel.h"
 
-ImageAreaPanel::ImageAreaPanel () : before(NULL), after(NULL)
+ImageAreaPanel::ImageAreaPanel () : before(nullptr), after(nullptr)
 {
 
     set_border_width (2);