Fix scrolling to the bottom in wxTextCtrl::AppendText().

Fix scrolling to the bottom in wxTextCtrl::AppendText().

The old code used EM_LINESCROLL which could scroll too far if the caret wasn't
in the first line of the control. Instead of trying to compute the correct
number of the lines to scroll, just use WM_VSCROLL with SB_BOTTOM parameter
instead.

Closes #12123.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64571 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775