Use separate 'initial-bearing' property instead of reciprocal.

The reason is that on long distances the reciprocal value is the
final bearing value (seen from station's perspective), and we need
the initial bearing angle (seen from aircraft's perspective).

Add support for switching initial-bearing to the NavaidSelector module.
Add missing configuration for FlapsControl modules.
Add ETA module that computes ETA to given station.

Configure for left and right VORs and for Home.
Fix comment.
Fix HSI - don't paint range scale twice.
Add Property::operator= for convenience.
Use Property::operator= in VOR module.