spec: refactor AcDbMTextObjectEmbedded to SUBCLASS #4619

Job	Run time
macOS	7m 43s
linux (CFLAGS="-O1 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindin...	16m 37s
linux-cmake	2m 2s
mingw	30m 18s
linux (CC="clang")	13m 30s
mingw-cmake	3m 11s
linux (--enable-release)	35m 51s
mips32-big-endian	8m 55s
linux (--enable-gcov --disable-bindings --disable-docs CFLAGS="-O0 -g")	8m 45s
ppc64-big-endian	8m 18s
linux (--disable-write --disable-docs)	15m 45s
vs2019	5m 12s
linux (CFLAGS="-O2 -m32" --disable-bindings --disable-docs)	13m 29s
vs2019-ninja	4m 49s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	30m 48s
	3h 25m 13s

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