Implement Windowed Fullscreen

There are two kinds of fullscreen. One where you take over the whole
output the other where you just set the window size to the screen
size and get rid of decorations. The first one already existed,
implement the second which is more common for normal desktop apps.
Use an enum to consolidate all the fullscreen states.

src/window.rs

@@ -7,6 +7,7 @@ use MouseCursor;
 use Window;
 use WindowBuilder;
 use WindowId;
+use FullScreenState;
 use native_monitor::NativeMonitorId;
 
 use libc;
@@ -59,10 +60,22 @@ impl WindowBuilder {
     /// Requests fullscreen mode.
     ///
     /// If you don't specify dimensions for the window, it will match the monitor's.
+    /// Disables fullscreen windowed mode if set.
     #[inline]
     pub fn with_fullscreen(mut self, monitor: MonitorId) -> WindowBuilder {
         let MonitorId(monitor) = monitor;
-        self.window.monitor = Some(monitor);
+        self.window.fullscreen = FullScreenState::Exclusive(monitor);
+        self
+    }
+
+    /// Requests fullscreen windowed mode.
+    ///
+    /// Disables the non-windowed fullscreen mode if set
+    #[inline]
+    pub fn with_fullscreen_windowed(mut self, fullscreen: bool) -> WindowBuilder {
+        if fullscreen {
+           self.window.fullscreen = FullScreenState::Windowed;
+        }
         self
     }
 
@@ -107,8 +120,10 @@ impl WindowBuilder {
     /// out of memory, etc.
     pub fn build(mut self, events_loop: &EventsLoop) -> Result<Window, CreationError> {
         // resizing the window to the dimensions of the monitor when fullscreen
-        if self.window.dimensions.is_none() && self.window.monitor.is_some() {
-            self.window.dimensions = Some(self.window.monitor.as_ref().unwrap().get_dimensions())
+        if self.window.dimensions.is_none() {
+            if let Some(monitor) = self.window.fullscreen.get_monitor() {
+                self.window.dimensions = Some(monitor.get_dimensions());
+            }
         }
 
         // default dimensions
@@ -305,6 +320,12 @@ impl Window {
         self.window.set_maximized(maximized)
     }
 
+    /// Sets the window to fullscreen or back
+    #[inline]
+    pub fn set_fullscreen_windowed(&self, fullscreen: bool) {
+        self.window.set_fullscreen_windowed(fullscreen)
+    }
+
     #[inline]
     pub fn id(&self) -> WindowId {
         WindowId(self.window.id())
@@ -362,7 +383,7 @@ pub fn get_primary_monitor() -> MonitorId {
 }
 
 /// Identifier for a monitor.
-#[derive(Clone)]
+#[derive(Clone, PartialEq)]
 pub struct MonitorId(platform::MonitorId);
 
 impl MonitorId {