Improvements to SplitContainer including a drag bar background StyleBox. #78

Job	Run time
📊 Static checks / Code style, file formatting, and docs	32s
🍎 macOS / Editor (target=editor, tests=yes)	29m 41s
🐧 Linux / Editor w/ Mono (target=editor)	40m 21s
🍎 macOS / Template (target=template_release)	5m 17s
🐧 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)	45m 38s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	3m 53s
🤖 Android / Template (target=template_release)	30m 32s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	34m 44s
🌐 Web / Template w/o threads (target=template_release, threads=no)	3m 58s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	30m 7s
🏁 Windows / Editor (target=editor, tests=yes)	48m 14s
🐧 Linux / Template w/ Mono (target=template_release)	24m 31s
🏁 Windows / Template (target=template_release)	34m 52s
🍏 iOS / Template (target=template_release)	1m 25s
🐧 Linux / Minimal template (target=template_release, everything disabled)	1m 31s
🪲 Godot CPP / Build and test Godot CPP	7m 22s
	5h 42m 38s

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