Improve get_plot_data() for patchwork plots

[MattCowgill]

I want to be able to use get_plot_data() for patchwork plots, which are ggplots (or other grid objects). I have implemented a hacky and sub-optimal solution - for each subplot in the patchwork plot, we get the data using the existing method, then dplyr::bind_rows() all the data into one data frame. This is clumsy and ugly - I do not like the results.

An alternative would be to return a list of dataframes. Then, when saving in Shiny, it could either:

• save each DF as a CSV, combine into one ZIP
• save each DF as a worksheet in an Excel workbook (using openxlsx) rather than CSVs