Post size events to the event handler and not window itself.

Post size events to the event handler and not window itself.

Post the artificial wxSizeEvent to the window event handler and not window
itself in SendSizeEvent() as otherwise any event handlers wouldn't see this
event at all.

Closes #13066.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67285 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775