Insert a string to a textbuffer at the given position.
Insère une chaine à ube position donnée d'un textbuffer.
_textbuffer_ : ObjGtkTextBuffer : any textbuffer object already created.
_string_ : S : a string to be inserted.
_position_ : I : a position where the string will be insterted. If pos == nil, this function is the same as _gtkTextBufferInsert.
_length_ : I : length of text in characters. If nil, entire texte will set.
Return : ObjGtkTextBuffer : the same _textbuffer_ or nil if error.