Improved Implementation to use Tweakables

These changes provide several improvements:

  - Engine Thrust Control now directly changes the Engine Thrust Limiter tweakable.
  - Increase / Decrease initial thrust options removed, since this feature is provided by the tweakables.
  - general code cleanup to improve performance and reduce size
  - SRB control now caches the initial SRB thrust limiter to ensure proper operation even after a save and reload. This was necessary due to the fact that the engine thrust controller now changes the engine thrust limiter that was previously used to extract this value.
  - SRB acceleration display implementations are replaced by one single implementation.

Note however:
  - The changes to the .csproj files should not be applied as the path structure was changes during development.
  - Several of the changes might break compatibility
  - GUI is now broken due to the missing InitialThrust methods. I will look into that later and provide the changes in a separate pull request.