Update :tools/lookup +xwidget support

Remove :tools/lookup +xwidget. Provide
`+lookup-xwidget-webkit-open-url-fn` so that we can manually configure
Xwidget WebKit browser for `+lookup/online` or `+lookup/in-docsets`.

modules/tools/lookup/autoload/xwidget.el

@@ -0,0 +1,20 @@
+;;; tools/lookup/autoload/xwidget.el -*- lexical-binding: t; -*-
+
+(defvar +lookup--xwidget-webkit-last-session-buffer nil)
+
+;;;###autoload
+(defun +lookup-xwidget-webkit-open-url-fn (url &optional new-session)
+  (if (not (display-graphic-p))
+      (browse-url url)
+    (unless (featurep 'xwidget-internal)
+      (user-error "Your build of Emacs lacks Xwidgets support and cannot open Xwidget WebKit browser"))
+    (let ((orig-last-session-buffer (if (boundp 'xwidget-webkit-last-session-buffer)
+                                        xwidget-webkit-last-session-buffer
+                                      nil)))
+      (setq xwidget-webkit-last-session-buffer +lookup--xwidget-webkit-last-session-buffer)
+      (save-window-excursion
+        (xwidget-webkit-browse-url url new-session))
+      (with-popup-rules! '(("^\\*xwidget" :vslot -11 :size 0.35 :select nil))
+        (pop-to-buffer xwidget-webkit-last-session-buffer))
+      (setq +lookup--xwidget-webkit-last-session-buffer xwidget-webkit-last-session-buffer
+            xwidget-webkit-last-session-buffer orig-last-session-buffer))))