gui: Consolidate wallet display name to GUIUtil function

Instead of having the code for the wallet display name being copy and
pasted, use a GUIUtil function to get that for us.

src/qt/walletmodel.cpp

@@ -594,8 +594,7 @@ QString WalletModel::getWalletName() const
 
 QString WalletModel::getDisplayName() const
 {
-    const QString name = getWalletName();
-    return name.isEmpty() ? "["+tr("default wallet")+"]" : name;
+    return GUIUtil::WalletDisplayName(getWalletName());
 }
 
 bool WalletModel::isMultiwallet() const