Bitcoin-Qt: misc changes for sendcoins and paymentsever

sendcoinsentry: small clear() and setValue() changes
- clear(): clear all UI elements (for secure and insecure payments)
- setValue(): only modify UI elements, which need to be set (for secure
  or insecure payments)

sendcoinsdialog: display real failed address string
- display the real string (faulty address), which causes the valid address
  check to fail, instead of a stringified "nonsense" CBitcoinAddress

paymentserver: use own HTML-esc / add new header const
- make processPaymentRequest() use our own HTML-escaping function from
  GUIUtil
- make string "application/bitcoin-payment" a constant (below similar
  constant strings in the .cpp file)