Use wxPanel as controls parent in wrapsizer sample.

Use wxPanel as controls parent in wrapsizer sample.

Follow our own advice about never creating controls directly on the frame and
using an intermediate wxPanel in our own sample. This is better style and
makes it look better under Windows.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65239 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775