forward/back navigation with shift+backspace and backspace

script caches last loaded url -- currently no caching policy
naive caching of render layers for near-instant forward/back
handling evicted pipelines is currently broken

src/components/main/layout/layout_task.rs

@@ -141,6 +141,7 @@ impl LayoutTask {
                 }
             }
             RouteScriptMsg(script_msg) => {
+                debug!("layout: routing %? to script task", script_msg);
                 self.route_script_msg(script_msg);
             }
             ExitMsg => {