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