[imp] merge po: refactor and add hooks

This feature has long been discussed with a MP on launchpad for v8:
https://code.launchpad.net/~camptocamp/openobject-addons/trunk-refactor-po-merge-lep/+merge/216841

and with a corresponding external module for v7:
https://code.launchpad.net/~camptocamp/purchase-wkfl/7.0-merge-po-hooks-lep/+merge/216745