Don't send SELECTED events for an already selected item in wxGTK wxListBox.

Bring wxGTK in line with wxMSW behaviour and avoid sending the
wxEVT_COMMAND_LISTBOX_SELECTED events when the user clicks on an already
selected item.

Refactor wxMSW code to extract the logic to avoid such events into a reusable
in other ports wxListBoxBase::DoChangeSingleSelection() function. Also add
wxListBox::GTKOnSelectionChanged() to wxGTK to avoid having to make the new
function public just so that it could be called by GTK callback and make the
previously existing CalcAndSendEvent() protected as well.

This fixes a unit test failure in ListBoxTestCase::ClickEvents() under wxGTK.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65935 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/listbox.h

@@ -100,6 +100,8 @@ public:
     void GTKDisableEvents();
     void GTKEnableEvents();
 
+    void GTKOnSelectionChanged();
+
 protected:
     virtual void DoClear();
     virtual void DoDeleteOneItem(unsigned int n);