improve plot_layout_properties: correct slide width/height, add title (closes #577)

[markheckmann]

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. With the changes, a box with the proper slide height/width independent of device size is used. plot_layout_properties() also gains a 'title' parameter, which will add the layout name as the plot title. Defaults to FALSE, to not alter the old behavior.

Before

New