[Admin] High memory when using a lot of pageparts on a page

When adding a lot of pageparts then the admin consumes a lot of memory. Some custom pageparts contains a lot of fields due all different configurations. The high memory is caused because all forms for all pageparts are build on load. Maybe we can only build the form when we click on edit for a pagepart? This means the admin should be refactored so that no content will be lost on save.