_gtkTextFullInsert inserts a string in a TextFull object.
The position can be defined as :
- the nth char of the text content or the nth char of a given line (see below)
- the start of the given line if the nth char is nil or the nth char in the given line
- if line and char are nil, a flag will be used : the beginning of the text (POS_START), the end of the text (POS_END), the current cursor position (POS_CURSOR)