Make ascent/descent rates available to QML through the Backend interface.
Remark signals from qPrefDivePlanner is used because the diveplanner model
sets qPrefDivePlanner but does not issue its own signals.
Signed-off-by: jan Iversen <jan@casacondor.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>