Minimap: "North" indicator arrow for circle minimap

Related: #3730

This adds a simple, and small "North" indicator to the circular
minimap. The indicator is in a classical triangle-like arrow with a
little bit of shading to accentuate the shape and give it a little
bit depth. The indicator is stuck exactly at the edge as far outwards
as possible, and is not too intrusive but still easy enough to spot.