Ensure that frame is re-laid out when its toolbar is deleted.

Ensure that frame is re-laid out when its toolbar is deleted.

The code in wxFrameBase::SetToolBar() didn't work correctly when toolbar was
unset using SetToolBar(NULL) because the frame toolbar pointer was reset
before layout was done resulting in the frame not recognizing its (still
existing) toolbar child as one of its bars and so nothing was done at all when
the frame had a single child, as in the toolbar sample.

Correct this by carefully ensuring that the toolbar pointer is still set at
the moment of the layout but hide the toolbar to ensure that no place is
allocated for it.

Also mention that it is not necessary to call SetToolBar(NULL) at all if the
toolbar is being deleted anyhow in the sample as toolbar does this itself in
its destructor.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63007 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775