never add core_theme to THEMES

[hhaensel]

Currently, when theme() is called once, the coretheme is added to THEMES[].

This is a bit unfortunate, because you first have to remove it manually if you want to render a page without core_theme. Moreover, parallel pages with and without core_theme are difficult to maintain. As theme() is anyhow called in order to append all stylesheets to the page, we can rely on the keyword attribute. Currently it's always added and then removed by unique!() if it was present before. Moreover, I made it a named function make it more understandable.

[hhaensel]

Maybe we should push the coretheme before all other themes so that the user can overwrite it. And we could name the function stipplecore() instead of coretheme()