first crack at context-driven store

test/apps/preloading/src/routes/_layout.html

@@ -7,12 +7,13 @@
 </script>
 
 <script>
-	export let preloading;
+	import { preloading } from '@sapper/app';
+
 	export let child;
 	export let rootPreloadFunctionRan;
 </script>
 
-{#if preloading}
+{#if $preloading}
 	<progress class='preloading-progress' value=0.5/>
 {/if}