transactionview: add message() signal

- allow to use message() in transactionview by connecting to the
  message() signal in WalletView
transactionview: make exportClicked() use message()

- use message() for displaying success or failure of export
- rework the strings to be more detailed / informative
- additional small cleanups
Merge pull request #3159

9eb4ab6 transactionview: make exportClicked() use message() (Philip Kaufmann)
868d3ee transactionview: add message() signal (Philip Kaufmann)