Make the WebKit inspector available

Make the WebKit inspector available

* etc/NEWS: Document changes.
* src/xwidget.c (find_widget_at_pos)
(find_widget)
(find_widget_cb): New functions.
(struct widget_search_data): New structure.
(Fmake_xwidget): Enable web inspector for WebKit widgets.
(Fxwidget_perform_lispy_event): Use current focus instead of
hard-coded widget.
(xwidget_button_1, xwidget_button, xwidget_motion_or_crossing):
Use window at event position instead of the default widget.