replace blocks timeout with a promise

The timeout as a promise lets us update the toolbox during the same
render as the workspace so that the screen updates all together. This
also means the toolbox will update before the loading screen is
dismissed after loading, letting us avoid a second flash for the
toolbox to update.

src/containers/blocks.jsx

@@ -180,13 +180,19 @@ class Blocks extends React.Component {
     componentWillUnmount () {
         this.detachVM();
         this.workspace.dispose();
-        clearTimeout(this.toolboxUpdateTimeout);
+        if (this.toolboxUpdateTimeout) this.toolboxUpdateTimeout.cancel();
     }
     requestToolboxUpdate () {
-        clearTimeout(this.toolboxUpdateTimeout);
-        this.toolboxUpdateTimeout = setTimeout(() => {
-            this.updateToolbox();
-        }, 0);
+        if (this.toolboxUpdateTimeout) this.toolboxUpdateTimeout.cancel();
+        let running = true;
+        this.toolboxUpdateTimeout = Promise.resolve().then(() => {
+            if (running) {
+                this.updateToolbox();
+            }
+        });
+        this.toolboxUpdateTimeout.cancel = () => {
+            running = false;
+        };
     }
     setLocale () {
         this.ScratchBlocks.ScratchMsgs.setLocale(this.props.locale);