Drawio: Started browser drawing backup store system

Adds just the part to store image data, and remove on successfull save.
Alters save events to properly throw upon error.
Adds IDB-Keyval library for local large-size store.
For #4421

resources/js/services/drawio.js

@@ -1,17 +1,22 @@
 // Docs: https://www.diagrams.net/doc/faq/embed-mode
+import * as store from './store';
 
 let iFrame = null;
 let lastApprovedOrigin;
-let onInit; let
-    onSave;
+let onInit;
+let onSave;
+const saveBackupKey = 'last-drawing-save';
 
 function drawPostMessage(data) {
     iFrame.contentWindow.postMessage(JSON.stringify(data), lastApprovedOrigin);
 }
 
 function drawEventExport(message) {
+    store.set(saveBackupKey, message.data);
     if (onSave) {
-        onSave(message.data);
+        onSave(message.data).then(() => {
+            store.del(saveBackupKey);
+        });
     }
 }
 
@@ -64,9 +69,11 @@ function drawReceive(event) {
 
 /**
  * Show the draw.io editor.
+ * onSaveCallback must return a promise that resolves on successful save and errors on failure.
+ * onInitCallback must return a promise with the xml to load for the editor.
  * @param {String} drawioUrl
- * @param {Function} onInitCallback - Must return a promise with the xml to load for the editor.
- * @param {Function} onSaveCallback - Is called with the drawing data on save.
+ * @param {Function<Promise<String>>} onInitCallback
+ * @param {Function<Promise>} onSaveCallback - Is called with the drawing data on save.
  */
 export function show(drawioUrl, onInitCallback, onSaveCallback) {
     onInit = onInitCallback;

resources/js/services/store.js

@@ -0,0 +1 @@
+export { get, set, del } from 'idb-keyval';

resources/js/services/util.js

@@ -70,3 +70,14 @@ export function uniqueId() {
     const S4 = () => (((1 + Math.random()) * 0x10000) | 0).toString(16).substring(1);
     return (`${S4() + S4()}-${S4()}-${S4()}-${S4()}-${S4()}${S4()}${S4()}`);
 }
+
+/**
+ * Create a promise that resolves after the given time.
+ * @param {int} timeMs
+ * @returns {Promise}
+ */
+export function wait(timeMs) {
+    return new Promise(res => {
+        setTimeout(res, timeMs);
+    });
+}