change: Use style-dictionary instead of theo

We've used theo to generate tokens, it was a bit cumbersome to
manage global aliases and hard to define property based filters.

From now on we use amazon style-dictionary to generate the tokens which is more flexible.
In the same process we renamed the tokens to get a more meaningful naming convention for css // scss variables.

https://github.com/owncloud/owncloud-design-system/pull/1217
