EZP-23014: Make YUI config extensible

> https://jira.ez.no/browse/EZP-23014
> [Specifications](https://github.com/ezsystems/PlatformUIBundle/blob/master/docs/design/yui-semantic-configuration.md)

This PR adds SiteAccess aware semantic configuration for PlatformUIBundle. Goal is to enable 3rd party bundles to provide their own configuration, without having to modify PlatformUIBundle core settings.

## TODO
* [x] Define semantic configuration parsing
* [x] Define base YUI config in new semantic format
* [x] Map semantic config to internal settings, taking `requires` and `dependencyOf` merge into account
* [x] Update Twig extension test
* [ ] Add functional test for configuration parsing / DIC extension
Nice :)
don't forget to remove the corresponding step in the README