racket/gui gtk: make `on-demand' for menus work better

There appears to be no way to make `on-demand' work right for the
Unity window manager's global menu bar, since there's no notificiation
when the menu bar is clicked. We approximate the correct behavior
by calling `on-demand' when a containing frame loses the keyboard
focus, which might be because the menu bar was clicked; that may be
too late (because the menu has already been shown), but it should
work most of the time.

Closes PR 13347.

Relevant to PR 13395, but DrRacket will have to change to work
around the remaining limitations of `on-demand'.
fix doc typo

Closes PR 13353
revise README and "Getting Started" info

Addresses PR 13356