@markup will be inserted in its entirety and must be nul-terminated
and valid UTF-8. Emits the [signal@Gtk.TextBuffer::insert-text] signal,
possibly multiple times; insertion actually occurs in the default handler
for the signal. @iter will point to the end of the inserted text on return.
Inserts the text in @markup at position @iter.
@markup will be inserted in its entirety and must be nul-terminated and valid UTF-8. Emits the [signal@Gtk.TextBuffer::insert-text] signal, possibly multiple times; insertion actually occurs in the default handler for the signal. @iter will point to the end of the inserted text on return.