Set speed to the forward component when switching modes

When in Manual Mode, and pressing F5 to change to Set Speed Mode, the speed was set to the magnitude of the current one. With this patch, it's set to the projection of the speed onto the target vector (or onto the ship's forward vector if no target is set), which feels more intuitively correct.
data/ui/Languages.lua
data/ui/MainMenu.lua

Polish translation:
['Error']
['An error has occurred']
['Could not load game: ']
No longer need to declare our own asinh and atanh from VS2013 onwards.
Also stop copying and start referencing a vector3d instead!
Use always forward vector regardless of target, to set speed on mode switch

It didn't feel right when the velocity wasn't pointing to the target.
Merge remote-tracking branch 'fluffy/byebyefeet'
Merge remote-tracking branch 'pgimeno/initialspeed'