Fix Shift+TAB navigation from wxNotebook.

Fix Shift+TAB navigation from wxNotebook.

When the focus was on a tab label, Shift+TAB didn't move it out of the
notebook to the preceding control, as it should. Instead, focus moved to
the last control in the current notebook page.

This change fixes it, TAB and Shift-TAB cycling through controls now
behaves identically modulo direction.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64432 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775