Image startup hooks

[Brian Rice]

I have added a Dictionary registry of blocks to handle startup actions 
for various objects in the system; see src/mobius/image.slate. This 
relies on the fact that saveImageNamed: returns False normally, and 
True for a saved image being started up by the VM.

Two handlers were added:
(1) ExternalLibrary now maintains an openLibs Dictionary which the 
open/close methods maintain. The start-up action calls #open on each of 
these libraries (Attila or whoever, please modify it to be more robust 
as necessary).

(2) The REPL can set itself up for reset on startup with "repl 
resetOnStartup."

Enjoy!

--
Brian T. Rice
LOGOS Research and Development
http://tunes.org/~water/