Improvements to xwidget on macOS (bug#60703)

Improvements to xwidget on macOS (bug#60703)

* src/nsxwidget.m ()
([XwWebView initWithFrame:configuration:xwidget:])
(nsxwidget_init):  Fixed memory leaks: when sending an alloc
message to an object, send an autorelease message to any objects
we won't explictly release.
([XwWebView webView:didFinishNavigation:]): Second string to
store in 'store_xwidget_event_string' is "load finished" rather
than empty string.
([XwWebView webView:didStartProvisionalNavigation:])
([XwWebView webView:didReceiveServerRedirectForProvisionalNavigation:])
([XwWebView webView:didCommitNavigation:]): New functions.
(nsxwidget_webkit_estimated_load_progress): New function.
(nsxwidget_webkit_stop_loading): New function.
* src/xwidget.c (Fxwidget_webkit_estimated_load_progress): Call
'nsxwidget_webkit_estimated_load_progress' if we're on MacOS.
(Fxwidget_webkit_stop_loading): Call 'nsxwidget_webkit_stop_loading'
if we're on MacOS.
(syms_of_xwidget): Define symbol for function.
'xwidget_webkit_estimated_load_progress' if we're on MacOS.
* src/nsxwidget.h: Signature for functions
'nsxwidget_webkit_estimated_load_progress' and
'nsxwidget_webkit_stop_loading'.
* lisp/xwidget.el (xwidget-webkit-current-url): Message URL rather
than return value of 'kill-new' (which is always nil).