qml: wizard to check if wallet name is already used

was erroring at the very last moment previously

electrum/gui/qml/components/wizard/WCWalletName.qml

@@ -5,7 +5,7 @@ import QtQuick.Controls 2.1
 import org.electrum 1.0
 
 WizardComponent {
-    valid: wallet_name.text.length > 0
+    valid: wallet_name.text.length > 0 && !Daemon.availableWallets.wallet_name_exists(wallet_name.text)
 
     function apply() {
         wizard_data['wallet_name'] = wallet_name.text

electrum/gui/qml/qedaemon.py

@@ -91,6 +91,7 @@ class QEWalletListModel(QAbstractListModel):
             self.wallets = wallets
             self.endRemoveRows()
 
+    @pyqtSlot(str, result=bool)
     def wallet_name_exists(self, name):
         for wallet_name, wallet_path in self.wallets:
             if name == wallet_name: