Scratch-blocks: Support labels in the flyout

[tmickel]

Example of when this may be desirable:

Hopefully we may also be able to reuse these labels for something like #258.

With an option we should make these labels/categories "sticky", like the iOS contacts list or like this fiddle: https://jsfiddle.net/yZKea/ (although almost certainly not with that implementation :))

[thisandagain]

Depends on #21

[tmickel]

@rachel-fenichel The infrastructure for this has landed, right? Assigning @carljbowman for any further design work.

[rachel-fenichel]

Yup, you've now got labels and buttons with arbitrary callbacks. can go anywhere blocks can go, and it's styled with CSS.

[carljbowman]

Looks like we have the framework for this feature.

Closing this issue until we have a specific use case, possible something we can explore in the extensions model.

Palette concept sketches that highlight possible use cases:

[CatsAreFluffy]

Why is Operators a stamp?

[carljbowman]

@CatsAreFluffy - I should have clarified. Those icons were placeholders and the exact icons should not be taken literally. While sketching I used random icons just to get a sense of if the icons created a overwhelming visual complexity. That way I did not have to think about the specific iconography and could sketch quickly.