Inserts text into an EditableText object.
As with all character offsets, the specified
position may not be the same as the resulting byte offset, since the text is in a
a pointer to the EditableText object to modify.
a int indicating the character offset at which to insert the new text.
a string representing the text to insert, in UTF-8 encoding.
the number of characters of text to insert, in bytes. If the byte count of text is less than or equal to length, the entire contents of text will be inserted.