system: enhance MarSystem and MarControl interface

* Let simple accessors return const references to members.

* Let MarControl store name without type separately.

* Add methods to MarSystem to find sub-systems and sub-controls
  with paths containing names only (without types).
realtime: improve OSC communication

* Make it completely real-time safe (no unnecessary dynamic allocation).

* Use control paths without types.
  (e.g. system-name/system-name/control-name)