Fix wxGenericTreeCtrl::ScrollTo() for all ports, not just wxOSX.

When scrolling down, make the item being scrolled into view completely visible
instead of just showing its top part.

The fix was already used for wxOSX but not for the other ports for some
reason, do use it everywhere as this code is generic and behaves in the same
way in all ports.

Also fix the wrong comments about scrolling direction.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76009 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775