Communication: Xpcc: make Identifier runtime writable

This might need to be reverted.
Right now it is necessary to enable runtimeid components.
However, before merging into develop, the whole concept
of runtimeif components should be reconsidered.
Tools: SystemDesign: add Component.getDescendantsWithId method
Tools: SystemDesign: enable runtimeid components

Runtimeid components allow to specify the implementation
of an abstract component at runtime.
The whole concept is still a little bit rough around the edges.
Thus before merging this into develop, the whole
concepts needs to be evaluated, tested and imporoved.