GTK Combobox and Listbox aren't realized until actually

GTK Combobox and Listbox aren't realized until actually
visible, so a list_item created before that would
receive a default style instead of the control's style.

Instead of calling ApplyWidgetStyle() which would
iterate all items, only the newly created item will
receive the current style to speed things up.

Thanks to Andreas Pflug.  Closes 984861

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@29032 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775