Theming for outline expanders for GtkTreeView

[Darin Adler <darin eazel com>]

Chris Lahey was just asking me a bunch of questions about outline expanders.
I wanted to mention the set of things we probably need to theme them
properly. One goal here is to be able to have really nice triangle-style
expanders like the Macintosh ones that work even better than on Macintosh in
some themes. And we could have the simple +/- ones in other themes.

Here are the issues. I hope this is not too late for 2.0.

    - expander image in both open and closed state
    - expander image prelit for both open and closed state
    - expander image tracked for both open and closed state, and a flag to
say whether clicking on the expander tracks like a button or not (since +/-
might want to just take effect on button down)
    - simple animation sequence for going from open to close or vice versa
(since for the triangles, you want a few frames of rotating to happen for
the best look)
    - flag to say whether lines are preferred or not -- I guess some
applications might want to force lines even with the arrow style? Not sure
how to make that look good.

What should I do to help get this right for GTK 2?

[Things aren't so good in Nautilus 1.0, because we have two tree views (one
in the Tree sidebar panel and one in the Help sidebar panel) that use
expanders that look different from each other, and they are not themed at
all.]

    -- Darin