Avoid too many sash position changed events when splitter is resized.

Avoid too many sash position changed events when splitter is resized.

We could call SetSashPositionAndNotify() twice in a row which was probably
unexpected and possibly wasteful, if the user code did anything non trivial in
response to this event.

Just call it once after we're certain which sash position do we want to set.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69601 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775