Grob names must only be unique amongst grobs which
share the same parent in a gTree.
This function can be used to generate a specification for
a grob that includes the grob's parent's name
(and the name of its parent and so on).
For interactive use, it is possible to directly specify
a path, but it is strongly recommended that this function
is used otherwise in case the path separator is changed
in future versions of grid.