plot_layout_properties(): args layout accepts index (davidgohel#595)

Accept the layout index (see `layout_summary()`) as alternative to
the layout name (suggestion 2 in davidgohel#595).

NEWS.md

@@ -24,7 +24,7 @@ informative error message if the type is not present in layout (#601).
 - Add `ph_location_id()` as a new member to the `ph_location_*` family. It references a placeholder via its unique id (#606).
 - `plot_layout_properties()`: Now accepts the layout index (see `layout_summary()`) as an alternative to the layout name.
   Also prints the current slide's layout by default now, if not layout name is provided explicitly (#595).
-
+  
 ## Features
 
 - `layout_rename_ph_labels()` to rename ph labels (#610).