len bytes of
text at position
len is -1,
text must be nul-terminated and will be inserted in its entirety. Emits the “insert-text” signal;
insertion actually occurs in the default handler for the signal.
iter is invalidated when insertion occurs (because the buffer
contents change), but the default signal handler revalidates it to point to the end of the inserted text.
a position in the buffer
text in UTF-8 format
length of text in bytes, or -1