Add affordance to indicate users can navigate to attached value blocks

[cpcallen]

During user testing, it was noted in many cases that, even after discovering that they could use the right arrow key to navigate from a block to the first value input on that block, users would frequently fail to realised that they could use the right arrow key to navigate to the attached block. This frequently resulted in the user placing the marker instead of (as desired) opening the colour picker.

It was suggested that we should provide some kind of affordance to make it clear that they can navigate further. Ideas mooted included:

• Some kind of tooltip or other pop-up hint ("keep going ->"?).
  • Display of such hints could be disabled once the user had successfully navigated to an attached block.
• Having the enter key open a pop-up menu with options including "replace attached block" (i.e., marker workflow) and "edit attached block".
  • N.B. as proposed this conflicts with planned changes to Make placing blocks using the marker more intuitive #102.

This is especially important for inputs with simple reporter shadow blocks, which (in the zelos renderer) look quite like ordinary fields.

See also #105.

[cpcallen]

Based on post-testing conversations this is nominally a fairly high-piority item (I've categorised as P1), but as I'm not sure exactly what form this affordance should take I'm not sure how to proceed.

Update: I had misread my notes; it was #105 that was the fairly high-priority item.

[rachel-fenichel]

@cpcallen I think this belongs in the micro:bit test milestone.

[rachel-fenichel]

Fixed by #274

[cpcallen]

The "edit" context menu item added by #274 was subsequently removed, but this issue had also been addressed by adding helpful toasts in #230.