Allow username selection for delegations and witness vote requests

[quochuy]

Currently, if you send a requestDelegation or requestWitnessVote without a username (delegator) it displays an error "The current website is trying to send a request to the Steem Keychain browser extension for account @null which has not been added to the wallet."

It would be good to allow username to be null and show the drop down selector so the user can select which account to do the delegation with.

This would allow rewriting SteemConnect links to Steem Keychain that is pending in #102

[quochuy]

@eonwarped as you did #92 do you think you could implement the same thing for delegations and witness votes?