This is a short patch that do 2 things :

- Add !readonly in the right place of textedit.cpp to correct a bug (you can add a \n into a readonly textedit field!
- Add a callback to CL_Tab, in order to enable used to be told that the current TabPage has changed. It is named TabPage as the tabheader's, and is actually called by the last one (CL_TabHeader's func_page_selected is linked to CL_Tab's on_page_selected, and on_page_selected invoke CL_Tab's func_page_selected). You can change the name of the callback, or just say that no-one will need it except me and do not apply this part of the patch