minor enhancements for links: open Info link in new window; mouse-3 menu for buffers with links

["Drew Adams" <drew.adams@oracle.com>]

Some minor link enhancements for consideration after the release.

1. Define commands `Info-follow-nearest-node-new-window' and
`Info-mouse-follow-nearest-node-new-window'. These just follow a link in a
new Info window (clone). I bind these to `S-RET' and `S-mouse-2'. Both are
handy.

2. These could be generalized to follow any link whose destination Emacs
understood (e.g. URL). In buffers with links, the same key bindings
(`S-RET', `S-mouse-2') might be appropriate.

Is there such a generic follow-link command already (one for RET at point
and one for mouse click)? Wouldn't that be useful? In Info, it would follow
an Info link. It would recognize a URL and follow that. And so on,
recognizing as many types of links as possible, depending, for instance, on
the buffer mode.

3. In buffers that have links, provide a mouse-3 context menu for a link.

In a typical Web browser, you can right-click on a link and get a popup menu
appropriate to the link. In IE6 with the Google toolbar, for instance, I see
this as the link menu:

 Open
 Open In New Window
 Save Target As...
 Print Target
 ------------
 Cut
 Copy
 Copy Shortcut
 Paste
 ------------
 Add to Favorites...
 ------------
 Backward Links
 Cached Snapshot of Page
 Similar Pages
 Translate Page into English
 ------------
 Properties

Two menu items that it would be particularly good to have are Open In New
Window and Backward Links. In Info, Open in New Window would do the same
thing as `Info-follow-nearest-node-new-window'.

Backward Links is a Google thing that is sometimes quite handy. In Emacs
Info, it would open a window of links to other Info nodes that also linked
to the destination of the clicked link. The same functionality would be
available for the current node (as opposed to the target of a link): show
all nodes that link to it. Backward links for the current-node could be
bound to a key sequence.