Add Window::set_blur

Allow clients to request blur behind their window, implemented on
Wayland for now.

src/platform_impl/windows/window.rs

@@ -127,6 +127,8 @@ impl Window {
 
     pub fn set_transparent(&self, _transparent: bool) {}
 
+    pub fn set_blur(&self, _blur: bool) {}
+
     #[inline]
     pub fn set_visible(&self, visible: bool) {
         let window = self.window.clone();