qt: clean up signal handling in walletframe/walletview

Use proper signals everywhere.
Removes the need to store a pointer to the BitcoinGUI object in
the walletview.
Also removes the interdependence between setWalletModel / setBitcoinGUI.
Merge pull request #3151 from laanwj/2013_10_walletview

qt: clean up signal handling in walletframe/walletview