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/walletview.cpp

@@ -137,7 +137,7 @@ void WalletView::setWalletModel(WalletModel *walletModel)
 void WalletView::incomingTransaction(const QModelIndex& parent, int start, int /*end*/)
 {
     // Prevent balloon-spam when initial block download is in progress
-    if(!walletModel || !clientModel || clientModel->inInitialBlockDownload())
+    if (!walletModel || !clientModel || clientModel->inInitialBlockDownload())
         return;
 
     TransactionTableModel *ttm = walletModel->getTransactionTableModel();
@@ -207,6 +207,7 @@ bool WalletView::handlePaymentRequest(const SendCoinsRecipient& recipient)
     if (sendCoinsPage->handlePaymentRequest(recipient))
     {
         gotoSendCoinsPage();
+        emit showNormalIfMinimized();
         return true;
     }
     else