Make the initial sash position work in splitter sample.

Make the initial sash position work in splitter sample.

The initial splitter size must be set correctly when using gravity with
wxSplitterWindow as otherwise the sash would jump on first resize -- which was
exactly what happened in the splitter sample.

Add a SetSize() call to the sample with the comment explaining why is it
needed and also actually make it stick as the cached last size was not updated
before the splitter was split before.

Closes #9821.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69599 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775