Draw circle instead of home arrow, when at home.
Rename PropertyObserver::keep_going() to touch().
Fixes for HSI, layers ordering, black background under texts.
Fixes for EFIS - bigger speed and altitude black-boxes.
Show TCAS information on HSI.

Also refactor some colors to InstrumentAids.
Bigger GS and TAS numbers on HSI.
Longer smoothing time for magnetic orientation.
Read any type of property by linear/radial indicator.
Config for reading engines' temperature and propellers' pitch.
Smaler 'at home' circle in HSI.
Add Datatable instrument.
Fix things with SI::Temperature.

Remove degC and degF operators, leave only absolute ones - K (Kelvin)
and Ra (Rankine). Add convenience static functions for getting
Temperature from °C and °F. Search and replace manual conversions
with those convenience methods.
Update (repaint) some instruments only when input props change.
Fixes on HSI: draw TCAS layer under the course layer.
Catch UnsupportedUnit exceptions and log them.
Move text again down a little.
Config update.
Better computations of Mach, TAS, etc.

Also derive back total pressure from sensed IAS, if using ready made
IAS sensor. Otherwise - compute own IAS from total and static pressures.

Add Xefis::Convergence class.

Update config.
Add @margin on <item> element allowing margins around instruments.
Optimize instruments that use threaded painting.

Don't paint anything unless the instrument is shown.