[Qt] fix slot connection problems when no wallet is loaded

- guard 4 connect calls by an #ifdef when no wallet is loaded to fix "No
  such slot" warnings
- also add comments when #ifdef ENABLE_WALLET end
- fixes #5175

src/qt/bitcoingui.h

@@ -63,7 +63,7 @@ public:
     bool addWallet(const QString& name, WalletModel *walletModel);
     bool setCurrentWallet(const QString& name);
     void removeAllWallets();
-#endif
+#endif // ENABLE_WALLET
 
 protected:
     void changeEvent(QEvent *e);
@@ -163,7 +163,7 @@ public slots:
 
     /** Show incoming transaction notification for new transactions. */
     void incomingTransaction(const QString& date, int unit, const CAmount& amount, const QString& type, const QString& address);
-#endif
+#endif // ENABLE_WALLET
 
 private slots:
 #ifdef ENABLE_WALLET
@@ -183,7 +183,7 @@ private slots:
 
     /** Show open dialog */
     void openClicked();
-#endif
+#endif // ENABLE_WALLET
     /** Show configuration dialog */
     void optionsClicked();
     /** Show about dialog */