Don't consider hidden root item to be visible in wxGenericTreeCtrl.

This fixes wrong results returned by GetFirstVisibleItem() in the tree control
with wxTR_HIDE_ROOT style.

Closes #4482.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76940 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775