refactor(WMTheme): clean up detectOpenbox #545

Job	Run time
FreeBSD	11m 12s
MacOS	15m 17s
Linux	4m 9s
Linux build test (ubuntu-22.04, gcc, g++, ON)	1m 16s
Linux build test (ubuntu-22.04, gcc, g++, OFF)	57s
spellcheck	12s
Win32	10m 58s
Linux build test (ubuntu-22.04, clang, clang++, ON)	1m 20s
Win64	4m 53s
Linux build test (ubuntu-22.04, clang, clang++, OFF)	1m 17s
Linux build test (ubuntu-22.04, musl-gcc, musl-g++, OFF)	58s
Linux build test (ubuntu-20.04, gcc, g++, ON)	1m 19s
Linux build test (ubuntu-20.04, gcc, g++, OFF)	1m 29s
Linux build test (ubuntu-20.04, clang, clang++, ON)	1m 37s
Linux build test (ubuntu-20.04, clang, clang++, OFF)	1m 14s
Linux build test (ubuntu-20.04, musl-gcc, musl-g++, OFF)	2m 23s
Release	0s
	1h 0m 31s

Provide feedback