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/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;