1.15.0

• add: thread-safe lsl_last_error() function, returns a description of the last error
  (#75, Tristan Stenner)
• change: object handles in the C++ API use smart pointers internally.
  The smart pointer to the object can be retrieved with the handle() function
• change: The stream_info copy constructor creates a shallow copy.
  Use stream_info::clone() for the previous behavior (deep copy).
• change: replace more Boost libraries with C++11 counterparts (Tristan Stenner)
• change: speed up push_chunk_* calls by caching the sampling rate (Chadwick Boulay)
• change: postprocessing parameters are reset after dis- and re-enabling postprocessing (Tristan Stenner)
• fix: postprocessing now works even when flushing samples (Tristan Stenner)
• fix: pulling strings with embedded NULL chars now works (Tristan Stenner, Chadwick Boulay)
• fix: various build system and deployment fixes (Chadwick Boulay, Tristan Stenner)
• fix: building with MinGW now works (Tobias Herzke)
• fix: max_buflen documentation and edge cases (Chadwick Boulay)
• fix: preliminary support for deploying Qt6 apps with CMake (Chadwick Boulay, Tristan Stenner)
• fix: packages for Ubuntu 18.04 no longer depend on the unavailable libgcc_s1 package
  (Tristan Stenner, many thanks to Tobias Herzke for reporting the issue)