Simplify API by only using non-const connect callbacks (#142) #450

Job	Run time
Build for 32-bit	44s
Run static checkers	29s
Installation tests	28s
macOS	54s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer=""]	2m 44s
Build with make	3m 5s
Windows	4m 56s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="thread"]	3m 2s
Windows (MinGW64)	55s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="undefined"]	2m 52s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="leak"]	2m 40s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="address"]	4m 0s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer=""]	2m 47s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="thread"]	3m 11s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="undefined"]	3m 53s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="leak"]	2m 48s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="address"]	3m 4s
Build with CMake [Debug, clang-18, cmake-3.29, sanitizer=""]	4m 4s
Build with CMake [Release, gcc-7, cmake-3.13, sanitizer=""]	4m 18s
Build with CMake [Release, clang-12, cmake-3.13, sanitizer=""]	5m 34s
	56m 28s

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