[Merge] ~nteodosio/software-properties:ubuntu/master into software-properties:ubuntu/master

[Nathan Teodosio]

Thank you for the review, Robert!

I addressed your comments, except for moving the non-GUI logic to a separate file. This is because I just learned that the magic enablement also needs attachment, which I already fixed here, but, you will note, with some redundancy.

Once I get that into GIS too, I can come back here to make that part cleaner. 
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https://code.launchpad.net/~nteodosio/software-properties/+git/software-properties/+merge/434360
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