Grob: add set_x_parent() and set_y_parent()
Grob: add set_x_parent() and set_y_parent(), and get_x_parent() and get_y_parent(), for use when the axis is known at compile time, which is usually the case.
Separating these operations also paves the way to constrain them independently, as Han-Wen recently proposed in !136 (closed), but with less branching.
@hanwenn, if you make timely progress on !136 (closed), we'll have to negotiate who will rebase.