Rename methods/activities with Visual in them

While they may be only 'visual' in terms of influence/cell grid,
they all do update CenterPosition, which is essentially the
actual world position of the actor.
'Visual' would imply that it only affects the position where the
actor is drawn, which is inaccurate.
Furthermore, using the term 'Visual' here would make
naming future methods/properties related to visual interpolation
unnecessarily complicated, because that's where we might
need a real 'Visual(Only)Position'.

OpenRA.Mods.Common/Activities/Parachute.cs

@@ -43,7 +43,7 @@ namespace OpenRA.Mods.Common.Activities
 			if (nextPosition.Z < groundLevel)
 				return true;
 
-			pos.SetVisualPosition(self, nextPosition);
+			pos.SetCenterPosition(self, nextPosition);
 
 			return false;
 		}