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

src/qt/walletstack.h

@@ -67,7 +67,7 @@ private:
     bool bOutOfSync;
 
 public slots:
-    void setCurrentWallet(const QString& name);
+    bool setCurrentWallet(const QString& name);
 
     /** Switch to overview (home) page */
     void gotoOverviewPage();