Added ClutterText::insert-text and ClutterText::delete-text signals

Added ClutterText::insert-text and ClutterText::delete-text signals

* clutter/clutter-marshal.list: added new marshaller
* clutter/clutter-text.c
(clutter_text_class_init): added insert-text and delete-text signals
(clutter_text_insert_unichar): emits insert-text signal
(clutter_text_insert_text): emits insert-text signal
(clutter_text_delete_text): emits delete-text signal
(clutter_text_delete_chars): emits delete-text signal
(clutter_text_set_markup_internal): emits delete-text and insert-text signals
(clutter_text_set_text): emits delete-text and insert-text signals

http://bugzilla.openedhand.com/show_bug.cgi?id=1894