MessageTemplate API now in Core

With 4.4, the API is available in Core now -- so use it.

(The previous local version is actually conflicting with 4.4; so we have
to switch to the Core one -- with a sligtly different entity name -- in
any case...)
Merge pull request #75 from sfe-ev/MessageTemplate

BAO name changed and fixed, indeed, much more correct in 4.4