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)
Add network traffic graph
misc spelling / space / style fixes
Merge pull request #3091 from Diapolo/misc

misc spelling / space / style fixes
Merge pull request #2924 from sje397/TrafficGraph

[QT] Add network traffic graph to debug window
replace %zu with %"PRIszu" macro to prevent compiler warnings
Merge pull request #3093 from Diapolo/printf_format

replace %zu with %"PRIszu" macro to prevent compiler warnings
Merge pull request #3080 from Diapolo/sendcoins-paymentreq

Bitcoin-Qt: misc changes for sendcoins and paymentsever
Allow SendMessages to run partially without cs_main

SendMessages() tries to acquire a cs_main lock now, but this isn't nessecary
for much of its functionality. Move those parts out of the locked section,
so they can always be performed, and we hold cs_main for a shorter time.
Run node deletions outside of cs_vNodes
Push down cs_main locking in ProcessMessage