Available Slot validation

[s2b]

Fluid should be able to validate if a filled slot is used by a component: Using <f:fragment> for a component that doesn't define a slot with the fragment's name should throw an exception. This will add a sanity check for template authors.

It should also be possible to disable this behavior to support component renderers that are unable to obtain the available slots:

1. Add property additionalSlotsAllowed to ComponentDefinition
2. Add method additionalSlotsAllowed() to AbstractComponentCollection