Use nullptr instead of NULL in the code and documentation

This is a combination of running clang-tidy with modernize-use-nullptr
check for some ports (GTK, X11, OSX) and manual changes to the ports for
which it couldn't be used easily (MSW, DFB) and also manually updating
the docs.

Also replace NULL with null or nullptr in the comments as this is more
consistent with the use of nullptr in the code and makes it simpler to
grep for the remaining occurrences of NULL itself.

And also use null in the assert messages.

Only a few occurrences of "NULL" are still left in non-C files, mostly
corresponding to unclear comments or string output which it might not be
safe to change.

samples/thread/thread.cpp

@@ -343,7 +343,7 @@ wxEND_EVENT_TABLE()
 
 // My frame constructor
 MyFrame::MyFrame(const wxString& title)
-       : wxFrame(NULL, wxID_ANY, title)
+       : wxFrame(nullptr, wxID_ANY, title)
 {
     m_oldLogger = wxLog::GetActiveTarget();
 
@@ -381,7 +381,7 @@ MyFrame::MyFrame(const wxString& title)
 
     m_nRunning = m_nCount = 0;
 
-    m_dlgProgress = NULL;
+    m_dlgProgress = nullptr;
 
 #if wxUSE_STATUSBAR
     CreateStatusBar(2);
@@ -592,7 +592,7 @@ void MyFrame::OnStartThread(wxCommandEvent& WXUNUSED(event) )
 
 void MyFrame::OnStopThread(wxCommandEvent& WXUNUSED(event) )
 {
-    wxThread* toDelete = NULL;
+    wxThread* toDelete = nullptr;
     {
     wxCriticalSectionLocker enter(wxGetApp().m_critsect);
 
@@ -740,7 +740,7 @@ void MyFrame::OnClear(wxCommandEvent& WXUNUSED(event))
 
 void MyFrame::OnUpdateWorker(wxUpdateUIEvent& event)
 {
-    event.Enable( m_dlgProgress == NULL );
+    event.Enable( m_dlgProgress == nullptr );
 }
 
 void MyFrame::OnStartWorker(wxCommandEvent& WXUNUSED(event))
@@ -778,7 +778,7 @@ void MyFrame::OnWorkerEvent(wxThreadEvent& event)
     if ( n == -1 )
     {
         m_dlgProgress->Destroy();
-        m_dlgProgress = (wxProgressDialog *)NULL;
+        m_dlgProgress = nullptr;
 
         // the dialog is aborted because the event came from another thread, so
         // we may need to wake up the main event loop for the dialog to be
@@ -932,7 +932,7 @@ wxThread::ExitCode MyThread::Entry()
         {
             wxCriticalSectionLocker locker(wxGetApp().m_critsect);
             if ( wxGetApp().m_shuttingDown )
-                return NULL;
+                return nullptr;
         }
 
         // check if just this thread was asked to exit
@@ -947,7 +947,7 @@ wxThread::ExitCode MyThread::Entry()
 
     wxLogMessage("Thread finished.");
 
-    return NULL;
+    return nullptr;
 }
 
 
@@ -980,7 +980,7 @@ wxThread::ExitCode MyWorkerThread::Entry()
 
 #if TEST_YIELD_RACE_CONDITION
     if ( TestDestroy() )
-        return NULL;
+        return nullptr;
 
     wxThreadEvent event( wxEVT_THREAD, WORKER_EVENT );
 
@@ -1011,7 +1011,7 @@ wxThread::ExitCode MyWorkerThread::Entry()
     wxQueueEvent( m_frame, event.Clone() );
 #endif
 
-    return NULL;
+    return nullptr;
 }
 
 
@@ -1072,7 +1072,7 @@ wxThread::ExitCode MyGUIThread::Entry()
     }
 
     // now remove the thread-specific thread target
-    wxLog::SetThreadActiveTarget(NULL);
+    wxLog::SetThreadActiveTarget(nullptr);
 
     // so that this goes to the main window again
     wxLogMessage("GUI thread finished.");