Dropdown menus

[xiaq]

This is inspired by ptpython, whose completion menu looks better than that of elvish (which is modeled after zsh):

The "dropdown" feeling can be attributed to two factors: a different background color, and margins on the sides.

[krader1961]

FWIW, I didn't know about ptpython until I looked at that project in response to reading this issue. So thanks for bringing that project to my attention. Nonetheless, I don't think this issue has sufficient information to drive a change in the existing elvish behavior. Especially since four years have elapsed since this issue was opened and a lot has changed in that interval. The example in the problem statement produces decent, if different, output in a current elvish version. Could the output be better? Probably, but who gets to decide which is better? At the end of the day a lot of issues like this one require reasonable defaults plus the ability of the user to tweak it to suit their needs. Which seems to be loosely related to issue #1001.