Don't duplicate event sending code in wxGTK wxListBox.

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

include/wx/gtk/listbox.h

@@ -101,6 +101,7 @@ public:
     void GTKEnableEvents();
 
     void GTKOnSelectionChanged();
+    void GTKOnActivated(int item);
 
 protected:
     virtual void DoClear();