add layout_dedupe_ph_labels() to hanble duplicate placholder labels (close #589)

[markheckmann]

Closes #589.

Building on the suggestions by @Majid-Eismann, I added layout_dedupe_ph_labels() to handle duplicate placholder labels in layouts. By default, it will detect duplicate ph labels, but apply no changes. With action = "rename", it auto-renames duplicate ph labels and action = "delete" deletes them, only keeping their first occurence. If requested, output is printed to the console, informing the user about the changes applied to the placeholder labels. I also added the sample PPTX file ph_dupes.pptx with some duplicate placeholder labels (see code below).

Tests are included. The added file ppt_ph_dedupe_layout.R has 100% coverage increasing the total coverage to 85.4%

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Examples

Code from layout_dedupe_ph_labels() docs.

x <- read_pptx()
layout_dedupe_ph_labels(x)

No duplicate placeholder labels detected. 

file <- system.file("doc_examples", "ph_dupes.pptx", package = "officer")
x <- read_pptx(file)
layout_dedupe_ph_labels(x)

Placeholders with duplicate labels:
* 'ph_label_new' = new placeholder label for action = 'rename'
* 'delete_flag' = deleted placeholders for action = 'delete'
  master_name layout_name  ph_label ph_label_new delete_flag
1     Master1     2-dupes Content 7  Content 7.1       FALSE
2     Master1     2-dupes Content 7  Content 7.2        TRUE

layout_dedupe_ph_labels(x, "rename", print_info = TRUE)

Renamed duplicate placeholder labels:
* 'ph_label_new' = new placeholder label
  master_name layout_name  ph_label ph_label_new
1     Master1     2-dupes Content 7  Content 7.1
2     Master1     2-dupes Content 7  Content 7.2

PS. In case of squashing commits, please ensure that @Majid-Eismann's commit also gets credits. :)

[davidgohel]

In case of squashing commits, please ensure that @Majid-Eismann's commit also gets credits. :)

I am going to "squash and merge" and then I will add you and Majid-Eismann as contributor