First steps to use the native compass mixins

**Don't merge this, this is just a POC to start the discussion**

In the `_mixins.scss`-file there is a lot of code that could be replaced with native compass-mixins.

There a several answers that should be answered before the process can be automated:

1. Should we use the prefixed mixins in the seperate files and use native Compass-mixins in the c_mixins.scssc-file? Or should we use the native Compass-mixins directly in the seperate files?
2. Which mixin is more correct? For instance, have a look at the `ctb-clearfix`-mixin, shouldn't we replace this with `@include clearfix();` from Compass?