Don't ask for tx-hash before payment method selected

[ligi]

this is wrong - we cannot ask for the transaction hash before the payment method is selected or the order is placed.
This bad flow was introduced with the security fixes for #8 - the problem is when fixing the problem there where obstacles to get an input field in the.

I do not think the user should enter the TX-Hash at all

I propose the following:
the user just selects the currency type:

then we show a QR (ERC-681) and a Web3Connect link (or js call) to trigger the payment

important is that the data-field gets a mandatory payment object ID - so we can prevent #8 and the sniping attack @pipermerriam pointed out

but this means in the beginning we can only take ETH and xDAI no DAI yet.
Later on (but IMHO not in wave 1 for DevCon) we should create a smart contract that accepts ETH and DAI with a function

EDIT: we can also take DAI by using the least significant bits of the value to encode the payment object ID (less than 5000WEI for DevCon - so in comparison to gas usage negligible)