FMA AT Engaged mode

A new multistate parameter is added to better read the AT engaged mode. It
would be possible in the future to extend the list of possible values for
other aircraft types with different engaged modes. Currently only the
modes for the B737-NG are considered.

One parameter is missing for the moment: AT HOLD.

It is very important that all these parameters are named consistently
across the various dataframes.

If you like it, before merging, let me know and I'll squash the 2 commits in 1.