Add Window::set_blur

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

src/platform_impl/web/window.rs

@@ -92,6 +92,8 @@ impl Inner {
 
     pub fn set_transparent(&self, _transparent: bool) {}
 
+    pub fn set_blur(&self, _blur: bool) {}
+
     pub fn set_visible(&self, _visible: bool) {
         // Intentionally a no-op
     }