default plasma theme and fallback

[Ivan Čukić]

> see attached patch.
+1

> personally i think that themes that rely on a certain subset of elements in
> the fallback theme remaining consistent are operating on a delusion.

It is not just that. The biggest problem I've had was that I had (per your 
suggestion) trimmed down Lancelot's themes to share as much elements as 
possible, so to reduce the number of SVGZs. Since most shipping-with-plasma 
themes are dark, and the default one was too, I removed all the elements from 
the other dark themes relying on the fact that those from the oxygen will be 
used. The renaming default->oxygen and air->default screwed all that up.

> in future, they can use FallbackTheme= if that's what they assume. and we
> need a techbase article on this :)
This would be a great solution for my case ;)

As for limiting the fallbacks to shipping-with-plasma themes, I don't really 
thing it is necessary, but it should be mentioned in the techbase article that 
we provide no mechanism of ensuring that the fallback theme exists, and that 
only the s-w-p ones should be used.

Cheerio.