Added parameter for the sole usage of the z-.. CSS classes

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | -
| Refs tickets  | -
| Fixed tickets | #1068
| License       | MIT
| Doc PR        | -

Instead of enabling the full compat_layer for the legacy css this config parameter allows to only enable the legacy css styling. All the other slowdown, memory consuming parts can be left out in that way for fully native Zikula 1.3.x modules.