introduce catamorphism for ui components

[nhaagen]

UI Components inductively built upon each other, i.e. components will contain other components.
It is, for some components, possible to access their substructure, but always in a specific manner; there is no generic way to get from the containing component to its children.

Now, @jeph864 is working on Activities and the idea arose to use forms/inputs to publish APIs (see #8746). What's missing right now is a simple way to fold down the inputs into e.g. some json-object, and while at it, this is true for all components.

Therefore, I implemented a mechanism on the componentHelper to walk down the object tree and apply operations to the components, storing the result in a property. This result does not need to stay a component, and you can see its effect quite clearly in the examples included here. Also, modification of the components' instances are perfectly feasible.

Components need to implement getSubStructure(): array to participate in the catamorphism; if they don't or return an empty array, recursion just stops and the component is considered a leaf-node.

This implementation comes with Container and Field\Group for now, but is explicitely meant for all UI Components.
I fiddled around wit the Panel, e.g., and it behaves just alike.