Bitcoin-Qt: fix GUI after initial multi-wallet patch

- adds 6 methods in BitcoinGUI to access some actions needed by the new
  WalletView class
- updates WalletView class to use these instead of trying to duplicate
  these
- cleanup walletview.{cpp/h} and remove all unneeded stuff
- this fixes problems with tabs toolbar (#2451) and export broken (#2436)
- more details in #2447