[FIX] orm.unlink(): cascade deletion of ir.model.data performed as admin
[FIX] orm.unlink(): cascade deletion of ir.model.data performed as admin

Also replaced all uid == 1 references with a ROOT_USER_ID constant, 
to make it more explicit.

akretion/openerp-server,akretion/openerp-server,akretion/openerp-server