Move beforeunload to plugin

plugins/before-unload.php

@@ -0,0 +1,33 @@
+<?php
+
+/** Display confirmation before unloading page if a form field was changed
+* @link https://www.adminer.org/plugins/#use
+* @author Jakub Vrana, https://www.vrana.cz/
+* @license https://www.apache.org/licenses/LICENSE-2.0 Apache License, Version 2.0
+* @license https://www.gnu.org/licenses/gpl-2.0.html GNU General Public License, version 2 (one or other)
+*/
+class AdminerBeforeUnload {
+
+	function head($dark = null) {
+		?>
+<script <?php echo Adminer\nonce(); ?>>
+// editChange is declared in functions.js
+// ajaxForm sets editChange to null on success
+
+addEvent(document, 'change', event => {
+	const el = event.target;
+	if (el.form && /post/i.test(el.form.method)) {
+		editChanged = true;
+	}
+});
+
+addEvent(document, 'submit', () => {
+	editChanged = null;
+});
+
+// all modern browsers ignore string returned from here
+onbeforeunload = () => editChanged;
+</script>
+<?php
+	}
+}