[openweathermap] Enhancement req - condition icon id

[Rossko57]

OpenWeatherMap API provides weather condition codes for current & forecasts.
Textual condition e.g "Light showers"
numeric condition ID e.g. "521"
icon ID e.g "12n"
The icon ID is from a less numerous graphic symbol set than the full condition, but additionally includes day/night coding. It is not straightforward to directly calculate an icon ID from the general condition ID.

The OWM binding uses this icon ID to calculate a URL and retrieve OWM images. But the URL or icon ID are not currently exposed to openHAB users.

Enhancement request - expose the icon ID as a string data channel in a similar manner to condition ID.
This would allow users to implement their own icon schemes.