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/forms/optionsdialog.ui

@@ -44,7 +44,7 @@
         </widget>
        </item>
        <item>
-        <layout class="QHBoxLayout" name="horizontalLayout_Main">
+        <layout class="QHBoxLayout" name="horizontalLayout_1_Main">
          <item>
           <widget class="QLabel" name="transactionFeeLabel">
            <property name="text">
@@ -62,7 +62,7 @@
           <widget class="BitcoinAmountField" name="transactionFee"/>
          </item>
          <item>
-          <spacer name="horizontalSpacer_Main">
+          <spacer name="horizontalSpacer_1_Main">
            <property name="orientation">
             <enum>Qt::Horizontal</enum>
            </property>
@@ -99,6 +99,36 @@
          </property>
         </spacer>
        </item>
+       <item>
+        <layout class="QHBoxLayout" name="horizontalLayout_2_Main">
+         <item>
+          <spacer name="horizontalSpacer_2_Main">
+           <property name="orientation">
+            <enum>Qt::Horizontal</enum>
+           </property>
+           <property name="sizeHint" stdset="0">
+            <size>
+             <width>40</width>
+             <height>20</height>
+            </size>
+           </property>
+          </spacer>
+         </item>
+         <item>
+          <widget class="QPushButton" name="resetButton">
+           <property name="toolTip">
+            <string>Reset all client options to default.</string>
+           </property>
+           <property name="text">
+            <string>&amp;Reset Options</string>
+           </property>
+           <property name="autoDefault">
+            <bool>false</bool>
+           </property>
+          </widget>
+         </item>
+        </layout>
+       </item>
       </layout>
      </widget>
      <widget class="QWidget" name="tabNetwork">

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();

src/qt/optionsdialog.h

@@ -36,6 +36,7 @@ private slots:
     void disableSaveButtons();
     /* set apply button and OK button state (enabled / disabled) */
     void setSaveButtonState(bool fState);
+    void on_resetButton_clicked();
     void on_okButton_clicked();
     void on_cancelButton_clicked();
     void on_applyButton_clicked();

src/qt/optionsmodel.cpp

@@ -60,6 +60,24 @@ void OptionsModel::Init()
         SoftSetArg("-lang", language.toStdString());
 }
 
+void OptionsModel::Reset()
+{
+    QSettings settings;
+
+    // Remove all entries in this QSettings object
+    settings.clear();
+
+    // default setting for OptionsModel::StartAtStartup - disabled
+    if (GUIUtil::GetStartOnSystemStartup())
+        GUIUtil::SetStartOnSystemStartup(false);
+
+    // Re-Init to get default values
+    Init();
+
+    // Ensure Upgrade() is not running again by setting the bImportFinished flag
+    settings.setValue("bImportFinished", true);
+}
+
 bool OptionsModel::Upgrade()
 {
     QSettings settings;

src/qt/optionsmodel.h

@@ -33,6 +33,7 @@ public:
     };
 
     void Init();
+    void Reset();
 
     /* Migrate settings from wallet.dat after app initialization */
     bool Upgrade(); /* returns true if settings upgraded */