Page title: `maxlength` option#281

Author: @Neil.here

Would be nice to have Blueprint-level constraints on a page’s title, such as maxlength.

Can essentially do this with a combination of page:before hooks, but would need to usepage.create:before and page.changeTitle:before. Even then, it wouldn’t have inline validation in the Panel.

Archived Idea: https://github.com/getkirby/ideas/issues/468

When setting up ‘one-pager’ sites, the child page titles often end up as the titles of the sections on the page. It would be really useful to be able to constrain the length.