Add option to make window "always on top" (#528)

* macOS: always_on_top

* Windows: always_on_top

* X11: always_on_top

* Stub set_always_on_top on other platforms

src/platform/android/mod.rs

@@ -323,6 +323,11 @@ impl Window {
         // N/A
     }
 
+    #[inline]
+    pub fn set_always_on_top(&self, _always_on_top: bool) {
+        // N/A
+    }
+
     #[inline]
     pub fn set_window_icon(&self, _icon: Option<::Icon>) {
         // N/A