* src/mathed/InsetMath.C: plaintext(): assert that this

	* src/mathed/InsetMath.C: plaintext(): assert that this
	method is never invoked (and tell why)
	* src/mathed/InsetMathFormulaMacro.C:
	* src/mathed/InsetMathHull.C: plaintext(): fix return value
	(= number of output characters)
	* src/mathed/InsetMathFormulaMacro.h
	* src/mathed/InsetMathHull.h: WS changes
	* src/mathed/InsetMathDim.C: implement plaintext() for math macros
	and add a FIXME that this code shouldn't needed!

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17319 a592a061-630c-0410-9148-cb99ea01b6c8