Major update to the AI code:

Major update to the AI code:
  * New features
    - More realistic descent paths
    - Separation during descent and approach
    - ATC approach controller (still silent)
    - inbound traffic flow will start immediately
  * Bug fixes
    - Properly handle vertical speed when on ground
    - Departing aircraft now wait for taxiclerance before moving
    - Traffic manager waits for proper weather initialization
    - Fixed instabilities in the preferential runway usage code
    - Fine tuning of waypoint following code.