PageCreateDialog: prevent tmp using actual page

[distantnative]

Description

@afbora Would appreciate your testing skills for this.

Summary of changes

• Page create dialog's temporary page object would use actual page if called new
• Not a perfect solution (ideal will be real in-memory virtual pages), but a lot less likely for a real-world page __new__

Changelog

Fixes

• Fixed issue where the page create dialog would use an existing page new instead just creating a temporary object
  Same Uuid is re-used every time when creating a child page #6640

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add changes & docs to release notes draft in Notion

[afbora]

I'm not sure what risks there might be in cloning an existing page rather than creating a new tmp page, but the issue fixed and everything seems to work fine.

LGTM 👍

[distantnative]

@afbora as @ovenum reports that the new page actually gets created but Kirby itself, this isn't ready as we probably haven't fully caught the bug.