bug#16354: 24.3.50; `M-o' conflict in Dired - change Omit mode to `C-x M-o'

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

dired-x.el has bound `M-o' to `dired-omit-mode', which is a toggle,
since Time Immemorial.  The facemenu commands were originally on
prefix key `M-g'.

Someone later had the bright idea to change `M-g' to prefix "go to"
commands.  And so, presumably without too much reflection about Dired X,
facemenu commands were moved to prefix key `M-o'.

So now you cannot put font-lock- or face-related commands on prefix
`M-o' in Dired, if you also use Dired X (which you should, of course).

`M-o' makes much more sense as a prefix key than as a toggle key.

I propose that we change the Dired X binding of `dired-omit-mode' to
`C-x M-o'.  Or to some other not-easily-repeatable key.  This toggle
does not need to be a quick key to use, at all.

In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2014-01-01 on ODIEONE
Bzr revision: 115827 eggert@cs.ucla.edu-20140101192741-bi5hb4xb4kdi2zpw
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/Devel/emacs/binary --enable-checking=yes,glyphs
 'CFLAGS=-O0 -g3' LDFLAGS=-Lc:/Devel/emacs/lib
 CPPFLAGS=-Ic:/Devel/emacs/include'