Split compound.* into multiple files by domain

By relocating according classes into compound_registry.*, compound_absorber_system.* and compound_emitter_system.*, issue #116 should be resolved.

- CompoundMovementSystem was kept in compound.*, because it is directly and only* linked to the CompoundComponent.
* (in relation to everything else that was located in compound.*)

- Note: The #includes in the new files where directly copied from compound.*. I couldn't put them up individually because I have little to no clue what each #include effectively references to. (Coming from Java (where the IDE pretty much told me exactly which "include" was missing and which one was unnecessary) didn't help either.) It didn't seem like that increased the compile time drastically in comparison to the time it took anyway already, but if someone feels like cleaning up, these #includes would be the right place for that ;-)
RNG shouldn't be needed for the absorber system