[sugar] Memory allocation indicators

[Benjamin M. Schwartz]

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The original inspiration for the Activity Ring was that the Ring could
serve both to indicate which activities were running and how much memory
they were using.  This was considered important in order to provide
feedback to prevent users from attempting to open many activities at once.
~ Typical Windows users have difficulty keeping track of which programs
they have open, and so wind up having many programs open simultaneously,
causing swapping even on machines with a great deal of memory.  This
problem is perhaps even worse on Mac OS X.  Since the original XO design
only had 128 MB of RAM, it was considered crucial that the UI provide
feedback to prevent an Out Of Memory situation.

With the Ring now being abandoned entirely, the UI will no longer contain
an indicator of memory usage.  The amount of memory is now 256 MB.  The
question is: do users need  to know how memory is being used, and if so,
what sort of indicator is appropriate?

Also, it would be easy for Rainbow to enforce a pre-set hard limit on
memory usage for each Activity separately.  I think that this is very
interesting, since it would allow us to ensure that OOM never happens, by
only launching an activity if all activities could still max out their
quota afterwards.  However, this reduces the functionality of the system,
by limiting the number of running activities to the worst-case scenario.
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