Added update script for migrating list_type plugin to CType

This was apparently changed with formhandler 2.1

class.ext_update.php

@@ -0,0 +1,59 @@
+<?php
+namespace Typoheads\Formhandler;
+
+use TYPO3\CMS\Backend\Utility\BackendUtility;
+
+class ext_update
+{
+    public function main()
+    {
+        $pluginsToMigrate = $this->countListTypePlugins();
+        if ($pluginsToMigrate > 0) {
+            $this->migrateListTypePlugins();
+        }
+        $remainingPluginsToMigrate = $this->countListTypePlugins();
+        if ($remainingPluginsToMigrate === 0) {
+            return "Success: Migrated $pluginsToMigrate formhandler plugins from list_type to CType. The forms should now show up again in the frontend.";
+        } else {
+            return "Error: $pluginsToMigrate formhandler plugins should have been migrated, but $remainingPluginsToMigrate are still left.";
+        }
+    }
+
+    /**
+     * Activate update script if any pre 2.1 plugins are in the database
+     *
+     * @return bool
+     */
+    public function access()
+    {
+        return $this->countListTypePlugins() > 0;
+    }
+
+    private function countListTypePlugins()
+    {
+        return $this->getDb()->exec_SELECTcountRows(
+            '*',
+            'tt_content',
+            'CType = \'list\' AND list_type = \'formhandler_pi1\'' . BackendUtility::deleteClause('tt_content')
+        );
+    }
+
+    private function migrateListTypePlugins()
+    {
+        $this->getDb()->exec_UPDATEquery(
+            'tt_content',
+            'CType = \'list\' AND list_type = \'formhandler_pi1\'' . BackendUtility::deleteClause('tt_content'),
+            [
+                'CType' => 'formhandler_pi1'
+            ]
+        );
+    }
+
+    /**
+     * @return \TYPO3\CMS\Core\Database\DatabaseConnection
+     */
+    private function getDb()
+    {
+        return $GLOBALS['TYPO3_DB'];
+    }
+}