Use const for constant local variables in wxWidgets headers

This avoids warnings from MSVS 2017 static analyzer when C++ core rule
set is activated.

Closes https://github.com/wxWidgets/wxWidgets/pull/819

include/wx/event.h

@@ -972,7 +972,7 @@ public:
     // propagation level value
     int StopPropagation()
     {
-        int propagationLevel = m_propagationLevel;
+        const int propagationLevel = m_propagationLevel;
         m_propagationLevel = wxEVENT_PROPAGATE_NONE;
         return propagationLevel;
     }