command registry

[ichthyo]

Interface was defined together with the first implementation of the Proc-Layer command handling frontend.
...we need this later, when we build the full command system, including serialisation and UNDO, because then we need a way of re-attaching de-serialised commands.

[ichthyo]

(In #148) test is written (and expected behaviour is defined), but still waiting for a preliminary impl. of the registry

[ichthyo]

documented expected behaviour by command-registry-test.cpp

[ichthyo]

(In #217) wrote a simple test, requesting some allocations and causing automatic de-allocations. Pending implementation

[ichthyo]

wrote a hashtable based implementation, but left out the custom allocator based implementation.