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

WalletView:
- add new signal showNormalIfMinimized()
- remove unneded checks from incomingTransaction()
- 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() to allow to check
  removals from WalletView
- 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()