Fix pointers on HSI to point to magnetic heading.
Fix radial indicator for negative input values.

Also rename 'normal' setting to 'reference'.
Get more values from FG concerning engines.
Detail fixes.
Batch changes.

* Incorporate Smoother into PIDControl.
* Use PropertyObserver in A/T.
* Detail fixes.
Allow configuring value range for axes in io/joystick.
Update config.
Deprecate signal_data_updated().
Fix a crasher: int argc must be a reference for some reason for Qt.

Also remove unused timers.
Fix nasty bug with returning from lambda instead of containing function.
Let io/flightgear send default values instead of random shit, if props are nil.
Use simple ParseError struct for speed.

Don't inherit Xefis::Exception, which analyzes stack, etc.
Multiple conditions per one EICAS status message.
Make io/flightgear set serviceable flags and other stuff for some sensors.
Support 'retained' data in io/link.

Retained property is not set to nil, when link is lost, instead its
last known value is kept.
Remove unnecessary aircraft weight parameter from Perormance Computer.
Change PID params for io/fly-by-wire to be settings, not properties.

Also signal FBW failure with serviceable flag.

Add rescue() method and update ModuleManager to call it in case of
exception from data_updated().
Link V/SPD serviceable flag to altitude serviceable flag.

This makes more sense.
Make FBW module use PropertyObserver.
Make FD module use PropertyObserver.