Don't send event from wxMSW::wxListCtrl::DeleteAllItems() if it did nothing.

wxEVT_COMMAND_LIST_DELETE_ALL_ITEMS should only be sent if the control hadn't
been empty before. Document this behaviour and adjust wxMSW to match the
other platforms.

Also document the return value better.

Closes #12336.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65381 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/listctrl.cpp

@@ -1464,7 +1464,9 @@ bool wxListCtrl::DeleteItem(long item)
 // Deletes all items
 bool wxListCtrl::DeleteAllItems()
 {
-    return ListView_DeleteAllItems(GetHwnd()) != 0;
+    // Calling ListView_DeleteAllItems() will always generate an event but we
+    // shouldn't do it if the control is empty 
+    return !GetItemCount() || ListView_DeleteAllItems(GetHwnd()) != 0;
 }
 
 // Deletes all items