Rework 'Fullscreen::Borderless' enum variant

This changes 'Fullscreen::Borderless' enum variant from
'Fullscreen::Borderless(MonitorHandle)' to
'Fullscreen::Borderless(Option<MonitorHandle>)'. Providing
'None' to it will result in picking the current monitor.

examples/fullscreen.rs

@@ -19,7 +19,7 @@ fn main() {
 
     let fullscreen = Some(match num {
         1 => Fullscreen::Exclusive(prompt_for_video_mode(&prompt_for_monitor(&event_loop))),
-        2 => Fullscreen::Borderless(prompt_for_monitor(&event_loop)),
+        2 => Fullscreen::Borderless(Some(prompt_for_monitor(&event_loop))),
         _ => panic!("Please enter a valid number"),
     });