Indicate what Scratchbook is when it's enabled.

Scratchbook is great, but very undiscoverable. I spent at least a year using Galaxy without knowing what it is.

I'd clicked the icon, but all that happens is that the icon changes, indicating it's "enabled". What did I just enable? No idea. So I turned it back off.

How about opening a Scratchbook window saying "You've enabled Scratchbook! Scratchbook is a neat way to view..." etc etc. Maybe include a "don't show me this again" button. Then I'd have left it on and used it a lot.

No implementation files or tests are named. Start by locating the Scratchbook enable/disable control and the Scratchbook window or view, then trace what happens when it is enabled. Done means enabling Scratchbook explains what it is and provides a way to suppress that explanation in the future.

Written by the indexing model from the issue text.