Add an option to InputfieldPage that allows new pages to be created regardless of duplicate titles

[JanRomero]

Someone in the forums seems to have a requirement to consistently create new pages from InputfieldPage. To my surprise, this isn’t what the field actually does. This PR adds the option forceCreate to make the field behave the way I’ve always interpreteted its descriptions. It also amends the description in the page editor to clearly state that it will prefer existing pages.

https://processwire.com/talk/topic/30187-auto-numbering-and-increment-value-of-the-field-of-new-child-pages-in-context-of-parent-page-reference-field/

I would have preferred to call the setting something like “prefer existing”, but I had to flip the logic to keep the old behaviour the default for backwards compatibility (checkboxes defaulting to checked are problematic because they don’t get sent when unchecked).

Thanks!