Add sleep() method to main.js.

Useful when the UI needs to "pause" for UX reasons. Can be called
from any component with `this.$root.sleep(ms)`.

lib/www/client/source/src/main.js

@@ -46,6 +46,12 @@ new Vue({
 
   methods: {
 
+    async sleep (ms = 0) {
+      return await new Promise( (resolve) => {
+        setTimeout( resolve, ms );
+      });
+    },
+
     markdown (value) {
       return markdown(value);
     },