[RFC?] Fullscreen mode with Alt+enter shortcut and dropdown option

* I didn't see a way to use non-hardcoded strings in the dropdown options, so I used placeholders
* The same with default dropdown option, for now it's the same as in the resolution dropdown, I'm not sure why, it's related to window_options_update but I don't know why it influences my new widget..
* I'm disabling the resolution dropdown when in the borderless fullscreen, as it's locked anyway. (by the way, it's strange how a widget can be both enabled and disabled at the same time)
* Note: changing the resolution in fullscreen may not be trivial, as SDL_SetWindowSize doesn't work there.

I'm not sure if the way I handled global config and osinterface_set_fullscreen_mode are correct, comments are welcome.