Fix forward declaration of `sf::Event` #405

Job	Run time
clang-format	15s
Windows MSVC Shared Release	2m 25s
Windows MSVC Shared Debug	2m 12s
Windows MSVC Static Release	3m 15s
Windows MSVC Static Debug	2m 58s
Windows ClangCL Shared Release	3m 52s
Windows ClangCL Shared Debug	3m 5s
Windows ClangCL Static Release	5m 47s
Windows ClangCL Static Debug	4m 18s
Windows Clang Shared Release	2m 2s
Windows Clang Shared Debug	2m 56s
Windows Clang Static Release	4m 3s
Windows Clang Static Debug	3m 35s
Linux GCC Shared Release	1m 11s
Linux GCC Shared Debug	56s
Linux GCC Static Release	1m 12s
Linux GCC Static Debug	1m 44s
Linux Clang Shared Release	1m 5s
Linux Clang Shared Debug	1m 21s
Linux Clang Static Release	1m 22s
Linux Clang Static Debug	1m 25s
macOS Clang Shared Release	1m 18s
macOS Clang Shared Debug	1m 22s
macOS Clang Static Release	1m 43s
macOS Clang Static Debug	1m 34s
	56m 56s

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