Implement ASET low altitude warning system

[MOARdV]

This is defined across several props (eg, LowALTIndicator) using a number of custom variables in CustomVariable_ALCOR_TabloIndicators (and maybe other places?).

Props depending on this feature

• MAS_Tablo_Alarm_LowAltitude (propTablo.xml)
• MAS_swRotary_4pos_AltitudeWarning (propRotarySwitch.xml)

[sovetskysoyuz]

Partially implemented in #368 (does not include switch to adjust threshold)