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.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 */