Fix bug help history overwrote by tree item selected call back

- Fixed the bug by making sure "_tree_item_selected" not get invoked when selecting class by code
- The bug will crash the editor which is not supposed to. ( It should only mesh up help history ) It might worth further investigation in case deeper bug is hidden.
- Crash happened at line: 
```
if (p_class==edited_class->get_text())
```