eCAL 6 C API Review

[KerstinKeller]

The C API is still based on eCAL 5. We need to review types / functions and their signatures.
This issue can help track this task.#

• Completely split C / C++ API files
  This probably requires some code duplication (E.g. some types like eCAL_Suscriber_Event are used by both C and C++ API.)
  Will also require clear naming schema ([core_c] Move implementation to lang folder. #1904)
• Rename files and folders ecal_c/ecal.h ([core-c] removed ecal_ prefix of public API header files #1939)
• Review C API and remove deprecated functions.... eCAL_Sub_Receive_Alloc / eCAL_Sub_Receive_Buffer_Alloc, eCAL_Sub_SetID, eCAL_Sub_SetAttribute, and more...
• SPubEventCallbackDataC / ... need desc_length field for descriptor. Even better, we create a struct for the SDatatypeInformationC
• eCAL headers are strictly speaking no longer available when using ecal C API. We should think about what header we might want for the C API (ecal_def.h / ecal_os.h) and create C versions of them...