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()). video_driver only supports direct3d9 and opengl, but this should be suficient for most users and for me, those are the only working ones ;)

I added them to the menu, but both of them need the user to restart Minetest to take the changes in effect.
opengl and direct3d9 are the most common ones and for me, they are also the only working ones.