Add utility functions and new xwidget commands

Add utility functions and new xwidget commands

Co-authored-by: Jaesup Kwak <veshboo@gmail.com>
* lisp/xwidget.el (xwidget-webkit-callback): Add case for
'response-callback' event.
(xwidget-webkit-download-dir): New variable.
(xwidget-webkit-save-as-file): New function.
* src/nsxwidget.m (XwWebView::decidePolicyForNavigationResponse):
Store download event.
* src/xwidget.c src/xwidget.h (store_xwidget_download_callback_event):
New function.