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/progressconnector.h

@@ -85,12 +85,12 @@ class ProgressConnector
     {
         retval = opStart.emit ();
         g_idle_add (ProgressConnector<T>::emitEndSignalUI, new sigc::signal0<bool> (opEnd));
-        workThread = 0;
+        workThread = nullptr;
     }
 
 public:
 
-    ProgressConnector (): retval( 0 ), workThread( 0 ) { }
+    ProgressConnector (): retval( 0 ), workThread( nullptr ) { }
 
     void startFunc (const sigc::slot0<T>& startHandler, const sigc::slot0<bool>& endHandler )
     {