omega/stamps.py: mechanism for custom colors in MultiStamp
omega/stamps.py: add a z-sort feature to MultiStamp

Can be helpful if you have a lot of overlapping symbols of differing
importance.

Also remove a commented-out line that we don't want to keep around.
omega/stamps.py: stroke *and* fill when drawing filled stamps

The stroke line around a stamp extends outside of the path coordinates by
0.5*linewidth, while the fill fills precisely inside the path coordinates.
Therefore, if we draw a filled stamp merely by filling the desired path, the
unfilled version of the stamp will be slightly larger. This becomes
distracting if we're drawing both filled and unfilled stamps and the linewidth
is nonnegligible.

Fix this by always stroking. Now filled and unfilled stamps have identical
outer edges. This looks much better.
omega/stamps.py: add a 'prepaint' feature to MultiStamp

This lets us call an arbitrary function just before painting a stamp item.
This can be used to add custom drawing, perform special setup of the Cairo
context, etc. Right now I'm using it to add special highlighting of certain
points. (This could be accomplished by adding extra quasi-dummy rows to
the data table, but then you're limited to the effects provided by the
data style, more or less.)
omega/styles.py: remove 'thickLine' as a style parameter

Instead put everything in terms of fineLine. Also set the linesize upon
initialization of the context, and don't override it in other places.

This changes the default look, which used to have data lines having
width=thickLine and axis tick lines having width=fineLine. So far I think this
change is beneficial.