Added new SplitContainer widget #704

Job	Run time
ios-sfml-graphics	52s
ios-sdl	2m 40s
android-sfml-graphics	1m 36s
android-sdl	4m 49s
linux	4m 58s
linux-latest-dev	4m 41s
linux-per-backend	5m 16s
linux-oldest	3m 5s
macos	1m 43s
windows	2m 10s
windows-clang	5m 27s
windows-oldest	6m 3s
windows-static-mt	4m 34s
nightly-build-windows-visual-studio	5m 49s
	53m 43s

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