Allow user to quickly review digitized images across multiple individuals

[droglenc]

A user (Nathan Stewart) would like to be able to quickly look at (review) his digitized images across multiple individuals. This can be accomplished with the following code (using a list of rds files in a directory as an example).

setwd()
tmp <- listFiles(".rds")
for (i in tmp) {
  showDigitizedImage(i)
  invisible(readline(prompt="Press [enter] to continue"))
} 

but it requires the user to click back to the console to continue to the next image. It would be better if I created a reviewDigitizedImages() function that took a list of rds file names, cycled through those but allowed the user to press "N" to go to the next image or "P" to go to a previous image. The new function should be able to rely on many of the internals for showDigitizedImage() along with some of the key press code in iSelectPoints().

[droglenc]

This is currently in the development branch version and will eventually be in v0.2.8.