Introduce the concept of an 'overlay activity'.

Introduce the concept of an 'overlay activity'.

This will be used for bottom-line editor prompts. Some of those are
triggered within the context of an existing activity (for example, if
you're reading a file and press the not-yet-implemented search key).
Others can be imposed from outside, e.g. pressing Alt+E while reading
a file. So there's a slightly fiddly mechanism to make the latter
work, and not treat it as two totally independent ESC and E
keystrokes (which would leave you looking at the Utilities Menu while
entering the name of a user to examine, instead of at the screen you
saw the username on).

The overlay activity is drawn on top of the topmost non-overlay
activity on the stack, at the bottom of the screen.