spec: refactor AcDbMTextObjectEmbedded to SUBCLASS #4615

Job	Run time
mips32-big-endian	1m 38s
linux (CFLAGS="-O1 -fno-omit-frame-pointer -fsanitize=address -fno-var-tracking" --disable-bindin...	1m 37s
linux (CC="clang")	1m 37s
linux (--enable-release)	1m 30s
linux (--enable-gcov --disable-bindings --disable-docs CFLAGS="-O0 -g")	1m 39s
linux (--disable-write --disable-docs)	1m 40s
linux (CFLAGS="-O2 -m32" --disable-bindings --disable-docs)	1m 36s
linux (CFLAGS="-O2 -fno-omit-frame-pointer -fsanitize=undefined -fno-var-tracking" --disable-bind...	1m 39s
mingw	1m 16s
linux-cmake	1m 36s
mingw-cmake	1m 23s
macOS	1m 46s
ppc64-big-endian	1m 42s
vs2019	1m 20s
vs2019-ninja	1m 22s
	23m 21s

Provide feedback