Provide "Restart Now" functionality

And use it for the plugin dialog and when changing the UI language.

Change-Id: Ic767837d2526409f7ec46d7e4612a1499f19459e
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/coreplugin/icore.h

@@ -153,6 +153,8 @@ public:
         MainWindowClosing,
     };
 
+    static void restart();
+
 public slots:
     static void saveSettings(SaveSettingsReason reason);