Preparations for coming features (#1818)

* removed unused convert_tz code
* added timestampable listener to support automatic updates of change columns
* added modified_at, billable and category timesheet fields
* new timesheet API query parameter modified_after

src/Repository/Query/InvoiceQuery.php

@@ -22,6 +22,12 @@ class InvoiceQuery extends TimesheetQuery
      */
     private $markAsExported = false;
 
+    public function __construct()
+    {
+        parent::__construct();
+        $this->setBillable(InvoiceQuery::STATE_BILLABLE);
+    }
+
     public function getTemplate(): ?InvoiceTemplate
     {
         return $this->template;