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/platform/linux/mod.rs

@@ -97,7 +97,7 @@ pub enum DeviceId {
     Wayland(wayland::DeviceId)
 }
 
-#[derive(Clone)]
+#[derive(Clone, PartialEq)]
 pub enum MonitorId {
     #[doc(hidden)]
     X(x11::MonitorId),
@@ -317,6 +317,14 @@ impl Window2 {
             &Window2::Wayland(ref _w) => {},
         }
     }
+
+    #[inline]
+    pub fn set_fullscreen_windowed(&self, fullscreen: bool) {
+        match self {
+            &Window2::X(ref w) => w.set_fullscreen_windowed(fullscreen),
+            &Window2::Wayland(ref _w) => {},
+        }
+    }
 }
 
 unsafe extern "C" fn x_error_callback(dpy: *mut x11::ffi::Display, event: *mut x11::ffi::XErrorEvent)