Improvements to SplitContainer including a drag bar background StyleBox. #89

Job	Run time
📊 Static checks / Code style, file formatting, and docs	22s
🍎 macOS / Editor (target=editor, tests=yes)	3m 12s
🍏 iOS / Template (target=template_release)	1m 50s
🍎 macOS / Template (target=template_release)	3m 26s
🐧 Linux / Editor w/ Mono (target=editor)	3m 40s
🤖 Android / Editor (target=editor)	4m 3s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	11m 34s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	3m 3s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	5m 57s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 19s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 6s
🏁 Windows / Editor (target=editor, tests=yes)	7m 9s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	7m 20s
🌐 Web / Template w/o threads (target=template_release, threads=no)	6m 34s
🏁 Windows / Template (target=template_release)	7m 25s
🐧 Linux / Template w/ Mono (target=template_release)	1m 6s
🐧 Linux / Minimal template (target=template_release, everything disabled)	53s
🪲 Godot CPP / Build and test Godot CPP	8m 22s
	1h 25m 21s

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