[FIX] base: module sequence number removed from XML data, moved to individual module manifests

Having the sequence number living outside the module itself
causes various problems at initialization when modules are
missing (e.g. if creating a database with `base` only).
It is also cleaner to have the module sequence in the
module manifest, like other module metadata.
A corresponding commit in the addons project adds the
`sequence` field in the manifest of all modules who
have a non-default sequence (the main apps).

bzr revid: odo@openerp.com-20120107041745-tik3iu1b2qs4ym85