Returns whether the character at this location is within an editable
region of text. Non-editable text is "locked" and can't be changed
by the user via W(TextView). This function is simply a convenience
wrapper around get_attributes(). If no tags applied to this text
editability, default_setting will be returned.
You don't want to use this function to decide whether text can be
inserted here, because for insertion you don't want to know whether
the char at iter is inside an editable range, you want to know whether
a new characer inserted here would be inside an editable range. Use
can_insert() to handle this case.