Support selecting which Kindle/eReader to send books to

[kevincrane]

Is your feature request related to a problem? Please describe.
I have two different Kindles, but I don't necessarily want to send a book/document to both of them every time I select "Send to eReader". I'd love a dropdown box to be able to select which reader to send a book to.

Describe the solution you'd like
I actually already put together a working solution! PR with the full implementation linked below. I've tested this on my own build and have verified that the correct devices show up in the dropdown menu and deliver to the correct eReader.
#3303

Thanks so much for taking a look!