Reuse wxListBoxBase::SetEvent() instead of duplicating its code in wxGTK.
Also get rid of the code checking for selection of the item with index -1:
this can't happen any more since r65865 which changed GTK_SELECTION_SINGLE to
GTK_SELECTION_BROWSE.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65936 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775