Add mainmenu settings for video_driver and fullscreen

The settings tab of the main menu lacks two important settings: video_driver (Important for those who want to use shaders, but get an error message when direct3d9 is used) and fullscreen (which uses core.get_screen_info()).

I added them to the menu, but both of them need the user to restart Minetest to take the changes in effect.
This line is too long, break it with two tabs.