Streamline ViewHelper API

[s2b]

Apart from the deprecation of renderStatic() and some minor changes under the hood, the AbstractViewHelper has been largely unaffected by Fluid v4. With the next major version, we might consider streamlining the user-facing ViewHelper API, while removing internal implementation details from the class (such as caches and some internal state).

We might also think about providing shortcuts to commonly used API in ViewHelpers and even a shortcut for getRequest() and some kind of API to make sure that the request is available. However, we should discuss before if we would like to make Fluid slightly "request-aware" by depending on the PSR interfaces for that.

[s2b]

In the long term, the abstract ViewHelper classes AbstractConditionViewHelper and AbstractTagBasedViewHelper should implement ViewHelperInterface directly and avoid extending the "god class" AbstractViewHelper. To achieve that, we need to move unnecessary logic from the ViewHelper to the framework (e. g. ViewHelperInvoker).