[Telepathy] change to file location specification

[Dafydd Harries]

For those unfamiliar with it: the FileFormats page on the Telepathy wiki[0]
describes the locations and formats of various Telepathy data files. It's the
only normative Telepathy document that's not the D-Bus interface
specification; at some point we might merge them into one document.

I've just changed that page as follows:

 - the XDG base directory specification[1] is used to locate files
 - ~/.telepathy is deprecated in favour of $XDG_DATA_HOME
 - $TELEPATHY_DATA_PATH is deprecated in favour of $XDG_DATA_DIRS

Rationale:

 - XDG is a Freedesktop.org standard, and Telepathy as a Freedesktop.org
   project should use it
 - we get free JHBuild compatbility, as it sets XDG_DATA_DIRS automatically --
   this is especially useful for Sugar development
 - users need to set fewer environment variables, assuming they are using XDG
   variables already

This is not the first time we've changed the rules regarding file locations,
but hopefully we can make it the last. Please voice any objections now.

A patch for dbus-python is already done; libtelepathy and Decibel will need
updating.

[0] http://telepathy.freedesktop.org/wiki/FileFormats
[1] http://standards.freedesktop.org/basedir-spec/basedir-spec-0.6.html

-- 
Dafydd