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/linux/mod.rs

@@ -302,6 +302,14 @@ impl Window {
         }
     }
 
+    #[inline]
+    pub fn set_always_on_top(&self, always_on_top: bool) {
+        match self {
+            &Window::X(ref w) => w.set_always_on_top(always_on_top),
+            &Window::Wayland(_) => (),
+        }
+    }
+
     #[inline]
     pub fn set_window_icon(&self, window_icon: Option<Icon>) {
         match self {