Make WIND_UPDATE_RATE configurable

Background
WIND_UPDATE_RATE is used to set how often a new target wind vector is chosen currently a new wind target vector is chosen every 15 seconds. Over the course of those 15 seconds the current wind vector will mix between the old and new targets.
Motivation
This can be too fast for my ocean wave simulator breaking the illusion if the new wind vector is wildly different then the old wind vector.
Proposal
Make WIND_UPDATE_RATE configurable by map scripts and or gadgets.
Side affects
Having a longer WIND_UPDATE_RATE would change the balance of wind generators. The lows and highs would be longer.

Aside
A widget could predict the target wind direction, and the wind strength over the next 15 seconds every time a new wind direction is generated based on how the wind speed and direction are changing, giving an unfair advantage.

Start with rts/Sim/Misc/Wind.h at WIND_UPDATE_RATE and rts/Sim/Misc/Wind.cpp around line 93 to understand how the 15-second target update is used. Trace the existing map-script and gadget configuration entry points, then verify that both can set the rate while the current default behavior remains unchanged.