Comprehensively rename to Mutter

Code:
All references in the code not related to themes, keybindings, or
GConf were changed from 'metacity' to 'mutter'. This includes, among other
things, strings, comments, the atoms used in the message protocol, and
the envvars used for debugging. The GConf schema file was reduced to
the 3 settings new to mutter.

The overall version was brought up to 2.27 to match current gnome.

Structure:
All files named '*metacity*' were renamed '*mutter*' with appropriate
changes in the automake system.  Files removed are
doc/creating_themes, src/themes, doc/metacity-theme.dtd,
metacity.doap.  These files will eventually end up in an external
gnome-wm-data module.

Installation location:
On the filesystem the mutter-plugindir was change from
$(libdir)/metacity/plugins/clutter to just $(libdir)/mutter/plugins.
The mutter-plugins.pc.in reflects these changes.

Note:
mutter.desktop.in and mutter-wm.desktop both continue to have
X-GNOME-WMSettingsModule=metacity set.  This allows
gnome-control-center to continue using libmetacity.so for
configuration.  This is fine since most the general keybindings and wm
settings are being read from /apps/metacity/* in gconf.

src/core/workspace.c

@@ -1,6 +1,6 @@
 /* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
 
-/* Metacity Workspaces */
+/* Mutter Workspaces */
 
 /* 
  * Copyright (C) 2001 Havoc Pennington