Indicates whether the line should be drawn before or after the track with
its index. This is mostly only relevant when gutter is used, since, then,
the position after a track is not the same as before the next
non-gutter track.
Resolves and positions all cells in the grid before creating it.
Allows them to keep track of their final properties and positions
and adjust their fields accordingly.
Cells must implement Clone as they will be owned. Additionally, they
must implement Default in order to fill positions in the grid which
weren’t explicitly specified by the user with empty cells.