Page title: `maxlength` option

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.

Can kill the design.