[sugar] Activity write space

[Michael Stone]

Bert,

> We have SUGAR_BUNDLE_PATH already - how about SUGAR_DATA_PATH or  

As I understood it, the point was that this variable may be unset in
some Sugar environments; hence, it shouldn't be "SUGAR_" prefixed.

> Would actually be nice to document the used vars somewhere ...

We've got a list tucked away in the Rainbow source code. See 

  http://tinyurl.com/2urddh

and please let me know if you know of important pieces we're missing. 

(Note: I'm hardly claiming this as "documentation", just pointing out
that the information is of immediate practical use to us)

Michael