Split up gammaray_probe

Use two libraries approach:
* Gammaray Core: core library
* Gammaray Probe: probe library (with hooks) linked to core library

Benefits:
* Plugins may link to gammaray_core directly
* We can write unit tests that link against gammaray_core
  Benefit: No hooking mechanisms triggered then
* Clearer separation of what belongs to hooking

It is mostly moving around of code, not too many semantic changes.
There *might* be issues wrt multithreading / missing locks.

TODO:
* Some things are marked with a TODO where I am unsure wrt correctness
* Please review those TODOs ACCURATELY!

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Milian, Volker: Please review.