Fix modal hidden focusable bug (#280)

This PR extracts the focusable array build into it's own method and includes a focus test. This also created the need to fire open and close sequentially so there was a refactor introducing promises so that an open call only happens after a close is complete. The impacts how opening modals from inside other modals works. Because of this refactor, it removes the need for callbacks since you can now wait for the promises to resolve. Lastly, a "working" check is done on click so that it's no longer possible to trigger a new modal open if one is in the middle of transitioning.

docs/_packages/modal.md

@@ -643,41 +643,55 @@ modal.init();
 modal.destroy();
 ```
 
-### `modal.open(key, callback)`
+### `modal.open(key)`
 
-Opens a modal provided the modal key and optional callback.
+Opens a modal provided the modal key and returns a promise that resolves to the modal object once the transition has finished.
 
 **Parameters**
 
 - `key [String]` A unique key that matches the value of a modal `data-modal` attribute.
-- `callback [Function]` (optional) A callback function to be run after the open process is completed.
+
+**Returns**
+
+- `Promise` The returned promise value will either be the `HTML object` of the modal that was opened, or `null` if a modal was not found.
 
 ```html
 <div class="modal is-closed" data-modal="modal-key">...</div>
 ```
 
 ```js
-modal.open('modal-key', () => {
-// Your custom code here...
+// Open modal
+modal.open('modal-key');
+
+// Run some code after promise resolves
+modal.open('modal-key').then((result) => {
+  console.log(result); // result = HTML Object || null
 });
 ```
 
-### `modal.close(returnFocus, callback)`
+### `modal.close(returnFocus)`
 
-Closes a modal and returns focus to trigger element with optional callback.
+Closes a modal and returns a promise that resolves to the modal object once the transition has finished. Optionally disable the return focus on trigger by passing `false` as the parameter.
 
 **Parameters**
 
-- `returnFocus [Boolean]` Whether or not to return focus on trigger once closed. Defaults to `true`.
-- `callback [Function]` (optional) A callback function to be run after the close process is completed.
+- `returnFocus [Boolean]` Whether or not to return focus on trigger once closed. Defaults to `true`. Helpful when opening a modal from another modal and you want to hold on to the initial activating trigger for later focus.
+
+**Returns**
+
+- `Promise` The returned promise value will either be the `HTML object` of the modal that was closed, or `null` if a modal was not found.
 
 ```html
 <div class="modal is-opened" data-modal="modal-key">...</div>
 ```
 
 ```js
-modal.close(true, () => {
-// Your custom code here...
+// Open modal
+modal.close();
+
+// Run some code after promise resolves
+modal.close().then((result) => {
+  console.log(result); // result = HTML Object || null
 });
 ```