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.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();