theme additions

[Richard Frith-Macdonald]

I have code to allow subclasses to replace the classes used for images  
and color lists by a theme, but I'm not sure they are really a good  
idea.

The thing is, if we are interfacing to some 'native' theming engine,  
the end user may change the theme in use by that system, which may  
change the colors and images we should be using.  Now with the current  
theme code, we can manage that by having the theme notice the change  
and deactivate itsself, change saved color lists and images on disk,  
then reactivate itsself so that it uses the new information, but  
that's all a bit clumsy (especially if the actual change is rather  
minor).

So all the update does is provide two methods which can be overridden  
to change the classes used by the theme to load the images and color  
lists.  The idea is that the NSImage and NSColorList subclasses used  
can then use the native theming engine where they need to, and can  
dynamically update the way they behave in response to a change in the  
native theme.

Sounds OK?