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