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
walletview: clarify 2 comments
Merge pull request #3160 from Diapolo/walletview

[Qt] walletview: make backupWallet() use GUIUtil::getSaveFileName()