Avoid events when implicitly selecting first wxBookCtrl page.

Avoid events when implicitly selecting first wxBookCtrl page.

The first page added to a wxBookCtrlBase-derived control is always selected,
even if "bSelect" argument of AddPage() was false. This is necessary because
a non-empty book control must always have a selection but the "selection
changed" event generated when doing it is unexpected.

Fix this by not generating any events when the first page is implicitly
selected.

Closes #12075.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65967 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775