Add sample code showing how to create a new wxEvent class

In the event sample, show how create and use a new
wxEvent-derived class.

Closes https://github.com/wxWidgets/wxWidgets/pull/1633

samples/event/makefile.gcc

@@ -31,7 +31,8 @@ EVENT_CXXFLAGS = $(__DEBUGINFO) $(__OPTIMIZEFLAG_2) $(__THREADSFLAG) \
 EVENT_OBJECTS =  \
 	$(OBJS)\event_sample_rc.o \
 	$(OBJS)\event_event.o \
-	$(OBJS)\event_gestures.o
+	$(OBJS)\event_gestures.o \
+	$(OBJS)\event_chessboard.o
 
 ### Conditionally set variables: ###
 
@@ -233,6 +234,9 @@ $(OBJS)\event_event.o: ./event.cpp
 $(OBJS)\event_gestures.o: ./gestures.cpp
 	$(CXX) -c -o $@ $(EVENT_CXXFLAGS) $(CPPDEPS) $<
 
+$(OBJS)\event_chessboard.o: ./chessboard.cpp
+	$(CXX) -c -o $@ $(EVENT_CXXFLAGS) $(CPPDEPS) $<
+
 .PHONY: all clean