Provided GetBestVirtualSize and DoGetBestSize implementations for

Provided GetBestVirtualSize and DoGetBestSize implementations for
generic and GTK+ wxScrolledWindow.

wxWindowBase::DoGetBestSize was returning a virtual size if there were child
controls, which was then used to set the scrolled window actual size.
Similarly, wxWindowBase::GetBestVirtualSize was returning
the actual window size; now we return the virtual size
for dimensions that have scrolling (e.g. vertical scrolling
direction), or the minimum/actual size for those that don't
(e.g. the maximum control width if there is no horizontal
scrolling).

This allows the scrolled window to be properly sized, whereas
before it would start off at the virtual size (giving
enormous layouts).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@32866 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775