Only recurse depth wise in `Tree::_count_selected_items` #21615

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 18s
🍏 iOS / Template (target=template_release)	34m 18s
🤖 Android / Template (target=template_release)	28m 27s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	21m 18s
🌐 Web / Template w/o threads (target=template_release, threads=no)	21m 1s
🐧 Linux / Editor w/ Mono (target=editor)	34m 54s
🍎 macOS / Editor (target=editor, tests=yes)	28m 34s
🐧 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)	37m 37s
🍎 macOS / Template (target=template_release)	21m 35s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	6m 20s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	27m 36s
🐧 Linux / Template w/ Mono (target=template_release)	23m 37s
🏁 Windows / Editor (target=editor, tests=yes)	41m 35s
🐧 Linux / Minimal template (target=template_release, everything disabled)	14m 36s
🏁 Windows / Template (target=template_release)	30m 5s
🪲 Godot CPP / Build and test Godot CPP	6m 39s
	6h 19m 30s

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