Doesn't work on any JSON file I have, it's a very naieve strategy provided by ethereumjs-wallet. Will need to raise its sophistication before deploying to production.
Fixes#1021
A top-right menu item now allows `Account Import`. It has a menu (with one item for now) that allows importing a private key string.
Errors are displayed, and a success navigates the user to their account list, where the imported account is labeled `LOOSE`.
