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)