bug#9054: 24.0.50; show source in other window

[martin rudalics]

> To support a text property or overlay by display-buffer would be nice.

If called `display-buffer', that property's value would appear like a
`display-buffer-alist' entry - a <PROPERTY, ACTION> pair that would be
run by `display-buffer' like an overriding action.

Users could inhibit that by adding to their `display-buffer-alist' a
<PROPERTY, USER-ACTION> entry where USER-ACTION nil would mean to not
run ACTION and USER-ACTION some other action would mean to run that
other action instead of ACTION.

> But a new user option is still needed for help-mode, so it could
> put the right value on text properties in the Help buffer.

Lars would like to rewrite help mode from scratch anyway, so I think
that would be a good occasion to do that.  In either case we should be
able to handle mouse clicks, keyboard input like "s" and "i", context
menus and external handling of *Help* navigation from another buffer in
a uniform way.

BTW: IMHO right clicking on a Lisp Object in Elisp mode with
`context-menu-mode' enabled should list

(1) Help for this object

(2) Manual entry of this object

_first_.  And Find Definition should allow to choose whether to do that
in another window/frame in the Context Menu.

martin