Terrain/RasterWeather: remove gettext() calls from ItemLabel() and ItemHelp()

Make the caller responsible for this.
Dialogs/RASP: disable the "time" control when "Terrain" is selected
Dialogs/RASP: remove special case for "Terrain"
GlueMapWindowOverlays: apply gettext() to RasterWeather::ItemLabel()
Terrain/RasterWeather: add struct MapInfo

Replaces the internal struct WeatherDescriptor.  Prepare for exposing
it.
Terrain/RasterWeather: add GetItemInfo()

Replaces ItemLabel() and ItemHelp().
Terrain/RasterWeather: add special case for "now" to [SG]etTime()

Missing piece from commit 013e654