Fixed dropdown height of wxChoice and wxComboBox controls.

Fixed dropdown height of wxChoice and wxComboBox controls.

When using comctl32.dll versions prior to 6.0 (e.g. with Win2k or no manifest file) the dropdown height of a wxChoice and wxComboBox would show all but one item and a vertical scrollbar was always visible. Fixed by reintroducing code that adds 1 to the total number of items. The code got dropped in r60553, added a comment to it in the hope that it will not be lost again.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69612 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775