Fixes for "Your First 2D Game" tutorial section 5: "The main game scene"

[acristin]

Section 5 of the first 2D game tutorial needed a few updates to match renames in the 4.0 beta.

Position2D -> Marker2D.

PathFollow2D "offset" with .progress_ratio, etc.

Issues reported in #5498

Please pay extra attention to the C# and C++ changes, as I haven't verified them yet.