[Qt] transactionview: make transactionview use message()

Related to #3157 and #3160 (same changes for addressbookpage and walletview).

Goal: Harmonize user experience for export of addresses, transactions and backup of the wallet!

![export_tx](https://f.cloud.github.com/assets/1419649/1414017/64e2b766-3e63-11e3-938d-028db94dbc2c.png)