Warn when user choses public key in "Import my key from file" dialogue

[nh2]

I just confused myself for 5 minutes when using the "Import my key from file" functionality from the initial setup window: I accidentally chose my pubkey instead of the private key, with the result that it simply appeared under the list of known contact keys, but not under "my keys".

It would be great if OpenKeychain could warn when you go via the "import my key" wizard and don't select a private key.