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.
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 'pgimeno/initialspeed'