Enter.key handling/detection is not implemented in PlainText powered blocks.
(See the More blocks).

We need to find a way to detect the Enter.Key pressed event on PlainText, and respond to it when the block that uses PlainText should trigger an action upon enter.key. (More or less the work we've done in RichText and AztecWrapper).

This is a desiderata for alpha. Move it to beta if we don't get to it in time.
Originally reported here: #316 (comment)