UTF8-slugs break blog and event engines

[oestrogen]

The blog and events engines do not work with pages that have UTF8 characters in their slugs.

There are two ways to reproduce this.

First way:
1. Create a new page and name it Blögg. The slug should become blögg.
2. Change the page engine to blog.
3. The page still looks like a template-based page.

Second way:
1. Create a new page and name it Blog. The slug should become blog.
2. Change the page engine to blog. The page should now be a blog page.
3. Change the page slug to blögg.
4. The page will now look like a regular template-based page.

[dordille]

This was fixed for the 1.4 branch in [1612] and the trunk in [1613].