Line now uses new style methods

Hi all,

This one was pretty straight forward, no problems. It did highlight for me the value of getting instance variables for styles since I changed a bunch of instance variable calls to method calls which look up style (I'll point that out in-line).

I also removed the `gui_opts` because it doesn't look like we're using those. I'll point that out in-line too.
Not strongly held, but at this point I'd probably be inclined to add a `set_point_a` for parity with `set_point_b`, and then have each of those methods take care of the `enclosing_box_of_line` at the end.

It's not a massive deal either way since the list of these accessors isn't liable to grow--a line isn't going to have many more dimensions and setters than we've already given it--but that'll make things neatly consistent.