Feat: Add ph_location_id() as a new member to the ph_location_* family (#606)

[markheckmann]

Add ph_location_id() as a new member to the ph_location_* family. It references a placeholder via its unique id (see #606, suggestion 4).

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Example

doc <- read_pptx()
doc <- add_slide(doc, "Comparison")
plot_layout_properties(doc, "Comparison")

Placeholder ids in green color in upper right corners.

doc <- ph_with(doc, "The Title", location = ph_location_id(id = 2)) # title
doc <- ph_with(doc, "Left Header", location = ph_location_id(id = 3)) # left header
doc <- ph_with(doc, "Left Content", location = ph_location_id(id = 4)) # left content
doc <- ph_with(doc, "The Footer", location = ph_location_id(id = 8)) # footer

file <- tempfile(fileext = ".pptx")
print(doc, file)
file.show(file) # may not work on your system

[davidgohel]

thanks @markheckmann