- Enhancement: Return a Promise for Editor's setCustomHandlers,

`loadFromString`, `loadFromDataURI` so known when ready and set
- Docs (Refactoring): Formally specify `Promise` resolve type;
    add `typedef` for dialog result object; add an
    `ArbitraryCallbackResult` type

editor/extensions/ext-markers.js

@@ -388,7 +388,7 @@ export default {
 
     /**
     * @param {"start"|"mid"|"end"} pos
-    * @returns {Promise} Resolves to `undefined`
+    * @returns {Promise<void>} Resolves to `undefined`
     */
     async function showTextPrompt (pos) {
       let def = $('#' + pos + '_marker').val();
@@ -426,7 +426,7 @@ export default {
     // callback function for a toolbar button click
     /**
     * @param {Event} ev
-    * @returns {Promise} Resolves to `undefined`
+    * @returns {Promise<void>} Resolves to `undefined`
     */
     async function setArrowFromButton (ev) {
       const parts = this.id.split('_');

editor/extensions/ext-placemark.js

@@ -240,7 +240,7 @@ export default {
     }
     /**
     * @param {Event} ev
-    * @returns {Promise} Resolves to `undefined`
+    * @returns {Promise<void>} Resolves to `undefined`
     */
     function setArrowFromButton (ev) {
       const parts = this.id.split('_');