Inserts len bytes of text at position iter.
If 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.
| this |
a `GtkTextBuffer` |
| iter |
a position in the buffer |
| text |
text in UTF-8 format |
| len |
length of text in bytes, or -1 |