Handle request_close in main routine.

 * Don't save project file twice.
WelcomeScreen refactoring.

 * Use TemplatePage class for file chooser (opener).
 * Add some symbols to TemplatePage.
WelcomeScreen refactoring (2).

 * Use TemplatePage class for project settings.
 * Remove deprecated and unused WelcomeScreen.add_page. All new template pages
   now has to use TemplatePage as base class.