Fix forward declaration of `sf::Event` #407

Job	Run time
clang-format	10s
Windows MSVC Shared Release	2m 51s
Windows MSVC Shared Debug	2m 9s
Windows MSVC Static Release	3m 36s
Windows MSVC Static Debug	3m 42s
Windows ClangCL Shared Release	4m 39s
Windows ClangCL Shared Debug	2m 53s
Windows ClangCL Static Release	8m 43s
Windows ClangCL Static Debug	7m 1s
Windows Clang Shared Release	2m 14s
Windows Clang Shared Debug	3m 23s
Windows Clang Static Release	4m 34s
Windows Clang Static Debug	3m 45s
Linux GCC Shared Release	55s
Linux GCC Shared Debug	54s
Linux GCC Static Release	1m 15s
Linux GCC Static Debug	59s
Linux Clang Shared Release	59s
Linux Clang Shared Debug	42s
Linux Clang Static Release	1m 31s
Linux Clang Static Debug	1m 28s
macOS Clang Shared Release	1m 44s
macOS Clang Shared Debug	1m 22s
macOS Clang Static Release	2m 9s
macOS Clang Static Debug	1m 35s
	1h 5m 13s

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