Bug 969672 - Handle declined engines in meta/global.
Bug 969672 - Part 2: add propagation of user-set declined engines.
Bug 969672 - Part 3: merge local and remote declined engines when processing a downloaded meta/global.
Bug 969672 - Part 4: add or remove engines from the declined list when we detect that they've changed.
Add declined engines to newly generated MetaGlobals.