Added setSpinButtonWidth function to SpinControl #750

Job	Run time
ios-sdl	2m 34s
android-sfml-graphics	2m 41s
android-sdl	6m 9s
ios-sfml-graphics	45s
linux	5m 17s
linux-latest-dev	2m 21s
linux-oldest	3m 3s
linux-per-backend	5m 29s
macos	1m 49s
windows	3m 21s
windows-clang	6m 2s
windows-static-mt	5m 59s
windows-oldest	6m 14s
nightly-build-windows-visual-studio	0s
	51m 44s

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