Bitcoin-Qt: tweak Qt walletXXX.cpp/h code

WalletView:
- add new signal showNormalIfMinimized()
- emit the new signal in handleURI() to fix a bug, preventing the main
  window to show up when using bitcoin: URIs

WalletStack:
- connect the showNormalIfMinimized() signal from WalletView with the
  showNormalIfMinimized() slot in BitcoinGUI
- rework setCurrentWallet() to return a bool
- add check for valid walletModel in addWallet()
- add missing gui attribute initialisation in constructor

WalletFrame:
- remove unused or unneded class attributes gui and clientModel
- add a check for valid clientModel in setClientModel()

General:
- small code formatting changes
Merge pull request #2872 from Diapolo/GUI_wallet

Bitcoin-Qt: tweak Qt walletXXX.cpp/h code
Bitcoin-Qt: extend some checks for valid model pointers
Merge pull request #2926 from Diapolo/model_checks

Bitcoin-Qt: extend some checks for valid model pointers