add a _settings function

Hi all

I have made some changes to the Interlaced theme:

- added a _settings function to select the CSS stylesheet variant like in the chameleon theme (in admin/system/themes).
- changed the directory structure to resemble the chameleon theme's directory structure, separating variants.

I have attached a zip archive with the changed Interlaced theme, which I named interlaced2 for testing.

I will now be beautifying the CSS some more (the navigation menu's indentation is too big per level in my opinion).
The screenshots need updating too.