bug#52973: Adding a few context-menu-mode commands

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  Philip Kaludercic <philipk@posteo.net>,
>   52973@debbugs.gnu.org
> Date: Wed, 12 Jan 2022 07:16:20 +0100
> 
> Juri Linkov <juri@linkov.net> writes:
> 
> > None of the proposed functions is specific to a mode.
> > When the global value of context-menu-functions
> > contains one of the proposed functions, right-clicking anywhere
> > will autoload every package where such context functions are defined.
> 
> Ah, I see.  I misunderstood how the context menu machinery works -- I
> thought it contained mode-specific stuff, and in addition, things that
> are relevant at the point.  That is, if you're over a text like
> "foo(1)", then there'd be a predicate to say that this might be a
> manual reference, and then you'd get the Man context menu.

How do we make sure stuff like "foo(1)" doesn't cause a lot of false
positives when applied in modes whose idea of what that means is very
different from Man-mode?

More importantly, if this stuff is supposed to be global, how do we
provide the users with a context-menu-mode which seamlessly does TRT
without asking the user to invest too much effort into populating
his/her menus?  I'm not aware of any popular application out there
which expects the users to customize context menus before they work as
expected, i.e. show relevant information in relevant places without
including a lot of irrelevant menu items.