A sendtocontract interface

[web3shannon]

A sendtocontract interface is always needed for Dapps. It can be like this:

1. Using a GET url to jump to a sendtocontract page.
2. The page contains a form and a confirm button.
3. Fill the form with the parameters in GET.
4. Parameters are the same as Qtum Core sendtocontract API:
   a. Contract Address (required)
   b. Data (optional, default empty)
   c. Amount (optional, default 0)
5. If the user does not have a wallet, guide him to create a wallet.