[PythonCAD] Sample patch for selection highlighting

[Stuart Brorson]

Art --

> The layer display should allow you to use the arrow keys to select which
> layer is active, as well as selecting the active layer by typing it.
> Also, when using the arrow keys, pressing the 'shift' key allows you to
> collapse or expand the displayed layer list. All these features came
> "free" because the display is built using the gtk.TreeView and
> gtk.TreeStore widgets.

Ok, I just added a couple of child layers to a drawing and tried this
out.  Yes, the arrow keys do move the selected layer up and down, and
I can also type in the name of the layer I want to select & it gets
selected.  

Some questions/issues:

*  Drawing in a new layer seems to draw only white.  The nice yellow
color which I am used to is not drawn.  All new layers only draw
white.  Is this the desired behavior? 

*  The new layer doesn't inherit the line width of the parent layer.
Any ideas about how to fix that?  I guess I can look at it
myself. . . .

*  I can add children to "top layer" , but I can't add a layer at the
same level.  From a philosophical perspective, what does this imply
about the top layer's role in a drawing?  I guess I am confused about
why one would have hierarchical layers, unless it's to draw the
leaders and notes to a drawing on a child layer.  But why aren't
multiple top layers possible (i.e. different drawings of different
components in a single file)?

Stuart