Add set_decorations method to Window (#365)

This has been stubbed on all platforms other than X11. The X11 implementation has also been
revised to toggle correctly, as it was previously only able to remove decorations.

src/platform/android/mod.rs

@@ -44,7 +44,7 @@ impl EventsLoop {
         MonitorId
     }
 
-  
+
     pub fn poll_events<F>(&mut self, mut callback: F)
         where F: FnMut(::Event)
     {
@@ -101,7 +101,7 @@ impl EventsLoop {
                     None
                 }
             };
-            
+
             if let Some(event) = e {
                 callback(event);
             }
@@ -288,6 +288,11 @@ impl Window {
         // Android has single screen maximized apps so nothing to do
     }
 
+    #[inline]
+    pub fn set_decorations(&self, _decorations: bool) {
+        // N/A
+    }
+
     #[inline]
     pub fn get_current_monitor(&self) -> RootMonitorId {
         RootMonitorId{inner: MonitorId}