Proposal: merge [S-tab] and [backtab]

[Paul Griepentrog <pgriepen@gmail.com>]

  Just a tidbit of computer evolution: the [backtab] key does not
exist on modern keyboards.  Of course, Emacs uses this key in
several modes (grep, compile, erc, ...).  So, in the place of
[backtab] within Emacs, [S-tab] is the conventional binding for a
poor man's backtab.  For convenience, some terminals (X,
Windows, and recently: NextStep) translate the keypress of
[S-tab] into [backtab] and nobody is the wizer.

Unless... you want to bind [S-tab] in a mode.  Then you find out
[S-tab] is NOT the key sequence you thought it was.  The binding
worked on X and Windows, but not at the terminal (or another
platform which did not link [S-tab] to [backtab]).  What
happened?!  Well, so you bind [backtab] as well.

My proposal is to universally translate [S-tab] and [S-iso-tab]
into [backtab] across all terminals.  At least then you can bind
[backtab] and know it will work across platforms and terminals.
Included in this proposal is removing obsolete bindings to
[S-tab] and [S-iso-tab] for modes within Emacs and updating the
documentation.  I include [S-iso-tab] since it is yet another
abused synonym for [backtab].

Note: this change will NOT ADD or REMOVE any bindings.  It is to
unify the keypresses of [S-tab], [S-iso-tab] to [backtab] where
they already exist.

I don't pretend to know the history of the keybindings or
specific terminals, but a uniform view of [S-tab] vs [backtab]
makes sense to me.

Thoughts?