[EW] Prevent silent overwrites when creating new files

Currently, if you create a new page with a name that already exists, it silently overwrites the existing one without any warning and you lose all the content.

We should either:
- Give the user a warning in a dialog
- Make it the same behaviour as when you paste a page with an existing name. Today, in that case, you get it as `{name}-copy`. For the create scenario, it could be `{name}-1`