several small Qt-related fixes

- make BitcoinGUI::showPaymentACK() use a reference for msg and use our
  own GUIUtil::HtmlEscape() function
- ensure QTimer usage in clientmodel is the same as in walletmodel
- remove an unneeded debug message in walletframe
- flag some parameters as unused in DebugMessageHandler()
- small code formatting changes
Merge pull request #3039 from Diapolo/Qt_misc

several small Qt-related fixes