Gettext, PCH++, ozEngine

- all oCore/*.cc and ozDynamics/*.cc include ozCore.hh/ozDynamics.hh, which is
  precompiled via ozCore_pch.hh/ozDynamics_pch.hh
- ozCore
  * JSON::read(), write() removed
  * Gettext class: generalisation of Lingua class, handles gettext .mo files
- ozEngine
  * new comonent to contain generic 3D engine building blocks in the future
- cmake
  * PCHs work from other directories