Add metadata to plot object

[flying-sheep]

It would be nice to be able to check programmatically if a given ggplot is a plot_grid and what layout it has.

As-is, you basically have to try if any of the elements is a GeomDrawGrob and then painstakingly reassemble the layout from geom_params$xmin/xmax/....

This would be useful to write an alternative to save_plot that infers the number of rows/columns using abovementioned metadata or information from facets.