walletview: make backupWallet() use GUIUtil::getSaveFileName()
walletview: make backupWallet() use GUIUtil::getSaveFileName()

- this allows removal of several Qt headers and makes use of a
  standardized function in GUIUtil
- adds selected path to the UI output

domob1812/crowncoin,Infernoman/crowncoin,Crowndev/crowncoin,Infernoman/crowncoin,syscoin/syscoin,domob1812/crowncoin,Crowndev/crowncoin,domob1812/crowncoin,Crowndev/crowncoin,domob1812/crowncoin,Infernoman/crowncoin,Crowndev/crowncoin,syscoin/syscoin,Infernoman/crowncoin,domob1812/crowncoin,Crowndev/crowncoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,Infernoman/crowncoin,domob1812/crowncoin,syscoin/syscoin,Infernoman/crowncoin,syscoin/syscoin,syscoin/syscoin,Crowndev/crowncoin