When column resizing is finished, after HDN_ENDTRACK notification there is also sent one (and last) HDN_ITEMCHANGING notification. We have to skip it to prevent from sending EVT_HEADER_RESIZING after EVT_HEADER_END_RESIZE because EVT_HEADER_END_RESIZE should be really the last one event in the sequence of resizing events (like it's assumed in wxGrid). Closes #16390.