bug#17644: 24.4.50; provide access to all mode-line lighters (e.g. those from C code)

[Drew Adams]

Elisp users can modify mode-line lighters for a major or a minor mode,
using `mode-name' and `minor-mode-alist'.  But they cannot modify (or
even remove) other lighters that are added by Emacs C code.  Please
enhance Emacs by making this possible (from Lisp).

For example, when the buffer is narrowed, the lighter `Narrow' appears.
It is neither a major-mode lighter nor a minor-mode lighter.  And there
is no way for a programmer to modify it from Lisp.

Please (a) document all such abnormal/extraneous lighters together,
somewhere in the Elisp manual and (b) provide a means for users to
access and modify them using Lisp.

In GNU Emacs 24.4.50.1 (i686-pc-mingw32)
 of 2014-05-25 on ODIEONE
Bzr revision: 117153 tsdh@gnu.org-20140525174054-vzeh4zeg00a1ley8
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/snapshot/trunk
 --enable-checking=yes,glyphs 'CFLAGS=-O0 -g3'
 LDFLAGS=-Lc:/Devel/emacs/lib 'CPPFLAGS=-DGC_MCHECK=1
 -Ic:/Devel/emacs/include''