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

Related to #3157 and #3159 (same changes for addressbookpage and transactionview).

Goal: Harmonize user experience for export of addresses, transactions and backup of the wallet!

![wallet](https://f.cloud.github.com/assets/1419649/1414039/4b926b92-3e65-11e3-8c5f-65c727020c5b.png)