Do give focus to the wxNotebook page when switching to it under MSW.

Do give focus to the wxNotebook page when switching to it under MSW.

Switching to the page but keeping the focus on the notebook itself makes it
difficult to use the UI from keyboard and is inconsistent with the behaviour
of native property sheets. Do restore the code to set the focus to the page as
the bug that resulted in a wrong radio button being selected when we did this
was apparently fixed elsewhere in the meanwhile because it doesn't happen any
more even with this change.

See #2268.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72657 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775