- 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/typedefs.js

@@ -10,10 +10,16 @@
 * @typedef {null|boolean|Float|string|GenericArray|PlainObject} JSON
 */
 
+/**
+* This should only be used when the return result from a callback
+*  is not known as to type.
+* @typedef {*} ArbitraryCallbackResult
+*/
+
 /**
 * @callback GenericCallback
 * @param {...*} args Signature dependent on the function
-* @returns {*} Return dependent on the function
+* @returns {ArbitraryCallbackResult} Return dependent on the function
 */
 
 /**