Fix forward declaration of `sf::Event` #404

Job	Run time
clang-format	12s
Windows MSVC Shared Release	1s
Windows MSVC Shared Debug	1s
Windows MSVC Static Release	1s
Windows MSVC Static Debug	1s
Windows ClangCL Shared Release	1s
Windows ClangCL Shared Debug	1s
Windows ClangCL Static Release	1s
Windows ClangCL Static Debug	1s
Windows Clang Shared Release	1s
Windows Clang Shared Debug	28s
Windows Clang Static Release	1s
Windows Clang Static Debug	1s
Linux GCC Shared Release	1s
Linux GCC Shared Debug	1s
Linux GCC Static Release	1s
Linux GCC Static Debug	1s
Linux Clang Shared Release	1s
Linux Clang Shared Debug	23s
Linux Clang Static Release	49s
Linux Clang Static Debug	1s
macOS Clang Shared Release	1s
macOS Clang Shared Debug	1s
macOS Clang Static Release	1s
macOS Clang Static Debug	1s
	2m 13s

Provide feedback