Must use GetAdjustedBestSize for getting child window sizes, since

Must use GetAdjustedBestSize for getting child window sizes, since
DoGetBestSize returns standard control size e.g. 100x100 on Windows,
and not an explicit size passed to it. This may be because the splitter
window isn't a true sizer and m_bestSizeCache isn't updated properly
(see window.h, GetBestSize: IsFullySpecified fails.)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@32557 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775