Allow to set custom transaction fee

[i3v]

When sending Ethereum, I constantly find that even "Economy" transaction fee is well above current Average transaction fee. And it takes mere seconds.

Although the difference between "what I would set" and your "Economy" isn't awful (within 0.5USD), I still find this quite annoying.

It would be great to have an option to specify the fee manually. (Yes, you would have to warn user that this is not very safe - the transaction might take hours and even get stuck...)