highlight current line when Emacs is idle

[Drew Adams]

This suggestion (for after the release) is a follow-on to my suggestion to
change the cursor to a box when Emacs is idle in order to more easily spot
it.

Suggestion: Optionally turn on `global-hl-line-mode' and highlight the
current line only when Emacs is idle. I use such an option in my code, and I
find it useful, for the same reason that changing to a box cursor when idle
is useful. I use a similar implementation for both (an input event ends the
highlighting).

I use a (configurable) idle-timer interval of several seconds - longer than
the timer interval for the cursor-type change. I also use a less-obtrusive,
yellow underline face for the highlighting (my background is light blue).

My guess is that more people might find hl-line useful with this option
added. The main reason some people don't like it is that it is very
intrusive and can be distracting.

In fact, I suspect that most people who use `global-hl-line-mode' (or
`hl-line-mode') would prefer this idle-only highlighting: there is usually
no need to continue highlighting the line after you have already noticed it.