Simplify API by only using non-const connect callbacks #441

Job	Run time
Run static checkers	32s
Build for 32-bit	27s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer=""]	2m 40s
Build with make	3m 22s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="thread"]	2m 53s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="undefined"]	2m 42s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="leak"]	2m 41s
Build with CMake [Release, gcc-14, cmake-3.29, sanitizer="address"]	2m 54s
Installation tests	23s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer=""]	2m 50s
macOS	1m 4s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="thread"]	3m 7s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="undefined"]	6m 37s
Windows	5m 55s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="leak"]	2m 51s
Windows (MinGW64)	54s
Build with CMake [Release, clang-18, cmake-3.29, sanitizer="address"]	3m 3s
Build with CMake [Debug, clang-18, cmake-3.29, sanitizer=""]	2m 48s
Build with CMake [Release, gcc-7, cmake-3.13, sanitizer=""]	4m 20s
Build with CMake [Release, clang-12, cmake-3.13, sanitizer=""]	5m 34s
	57m 37s

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