Fix bug with not selecting wxAuiNotebook page when its child was focused.

The code in OnChildFocusNotebook() handler only worked correctly if the page
itself was focused but not if the focus was given to one of its children --
which should still make the page itself current.

Closes #15471.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74769 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775