* src/output_plaintext.C: check line length before outputting

	* src/output_plaintext.C: check line length before outputting
	a word in front of an inset; make sensible use of plaintext()
	return value (IMHO totally broken before)
	* insets/insetfoot.[Ch]: add plaintext()
	* insets/insethfill.[Ch]: adjust plaintext(); make the
	number of characters determinitic; minor header cleanup
	* insets/insetbase.h: add comment on return value
	of plaintext() - before its meaning was unclear

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17200 a592a061-630c-0410-9148-cb99ea01b6c8