Add yellow arrow that points to the first portal of the current mission in case no waypoints of no missions are visible on the map

[Gamer1120]

An attempt was made here: https://github.com/Gamer1120/OpenBanners/tree/feat/yellow-arrow

It's a lot of effort for something I probably won't use much, so it's going on the "will probably fix in the very distant future, unless someone else picks it up first" list.