bug#25987: 25.2; support gcc fixit notes

[Eli Zaretskii]

> From: Tom Tromey <tom@tromey.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  25987@debbugs.gnu.org
> Date: Wed, 08 Mar 2017 11:44:59 -0700
> 
> David Malcolm (who wrote the gcc feature) pointed me at the Eclipse bug
> for this:
> 
> https://bugs.eclipse.org/bugs/show_bug.cgi?id=497670
> 
> It sounds like they're going to integrate it into their "quick fix"
> feature, which is an Eclipse thing.

Thanks.

Quick fixes work by presenting a kind of tooltip.  We should probably
have something similar, but it cannot be the only UI, to support TTY
and GUI users who don't like the mouse.