Add affordance to indicate users can navigate within blocks

[cpcallen]

During user testing, it was noted in many cases that the user did not realise they could use the right arrow key to navigate to the fields and inputs of the block the cursor was on.

It was suggested that we should provide some kind of affordance to make it clear that they can navigate within the block using the right arrow key. Ideas mooted included:

• Have the enter key move to the first field of the block, or open a menu of suitable actions (e.g. "replace this block", "edit block settings").
• Display a tooltip or other pop-up hint ("press right to access contents ->"?) to indicate it is possible to access the contents of the block.
  • Display of such hints could be disabled after the first time they are successfully applied.

See also #104.

[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.

[cpcallen]

It is planned to resolve this via an "edit contents" item in the context menu (see #132), which will be displayed when the user presses enter while focus is on a block (see #130).

[rachel-fenichel]

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

[kmcnaught]

Just to note that this could be resolved via an "edit contents" item in the context menu, or via a toast/notification, depending on the details of the flow (as per Matt's different options he's been experimenting with)