Bitcoin-Qt: add a Reset button to the options dialog

- a click on "Reset Options" sets all options to the default values by
  removing all stored settings (QSettings), loading the defaults and
  saving them as the new settings
- before the reset is executed the user is presented a confirmation dialog
- special casing was needed for StartAtStartup

src/qt/optionsdialog.cpp

@@ -177,6 +177,33 @@ void OptionsDialog::setSaveButtonState(bool fState)
     ui->okButton->setEnabled(fState);
 }
 
+void OptionsDialog::on_resetButton_clicked()
+{
+    if(model)
+    {
+        // confirmation dialog
+        QMessageBox::StandardButton btnRetVal = QMessageBox::question(this, tr("Confirm options reset"),
+            tr("Some settings may require a client restart to take effect.") + "<br><br>" + tr("Do you want to proceed?"),
+            QMessageBox::Yes | QMessageBox::Cancel, QMessageBox::Cancel);
+
+        if(btnRetVal == QMessageBox::Cancel)
+            return;
+
+        disableApplyButton();
+
+        /* disable restart warning messages display */
+        fRestartWarningDisplayed_Lang = fRestartWarningDisplayed_Proxy = true;
+
+        /* reset all options and save the default values (QSettings) */
+        model->Reset();
+        mapper->toFirst();
+        mapper->submit();
+
+        /* re-enable restart warning messages display */
+        fRestartWarningDisplayed_Lang = fRestartWarningDisplayed_Proxy = false;
+    }
+}
+
 void OptionsDialog::on_okButton_clicked()
 {
     mapper->submit();