v0.30

[basiliscos]

• [feature] added fltk-backend
• [feature, conan] enable_fltk option which add fltk-support
• [example] added /examples/ping-pong-fltk.cpp and
  examples/ping-pong-fltk_and_thread.cpp
• [bugfix] wx-backend building and testing
• [improvement, breaking] output directories are set to bin for cmake
• [improvement, breaking] actor_base_t::make_error() is marked as const

---

This discussion was created from the release v0.30.