Work on QtConsole Menu

* add new_frontend_factory, slave_frontend_factory callable arguments to MainWindow
  constructor, decoupling MainWindow and the IPythonApp a bit
* create *all* menu items in MainWindow.init_menubar, rather than some in the IPythonApp
* split init_menu_bar into methods for each menu, to ease organization
* attach all menu bar actions to the MainWindow as well, so that they are still
  accessible when Menu Bar is invisible
* add View menu for Fullscreen and Fonts
* organized menu items to match standard patterns
* Fewer platform-dependent menu items