Display proper slide width and height in plot_layout_properties(), add layout title

[markheckmann]

Just a small visual improvement for plot_layout_properties().

Currently, a box is drawn around the plotted placeholders which does not reflect slide width and height properly, as it depends on device size. I suggest to add a box with the proper slide height/width independet of device size. Also, I suggest to add the layout name as a plot title. See below.

Code changes here: https://github.com/davidgohel/officer/compare/master...markheckmann:officer:improve_plot_layout_properties

If okay, I will submit a PR.

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