For the target (compound) loon plot, determines all arguments (i.e. including the grobs) to be passed to `gridExtra::arrangeGrob()` so as to determine the layout in `grid` graphics.
l_get_arrangeGrobArgs(target)
a list of the named arguments and their values to be passed to `gridExtra::arrangeGrob()`.
the (compound) loon plot to be laid out.