[sugar] datastore use cases for activities

[Eben Eliason]

> Activity-provided file selection boxes are explicitly not supported by
> the current Bitfrost spec.

Why not?  Isn't this just what the security mechanisms make possible?
As long as the user explicitly chooses the items through a secure
dialog, what difference does it make if their choices are any photo or
any file at all?

As much as I'd prefer these choosers to be media sensitive and
targeted for correct content types, an example such as the upload
scenario in Browse isn't one we can ignore.

- Eben