Fix bug 3750: this code is a kludge and should be removed when tex2lyx is

Fix bug 3750: this code is a kludge and should be removed when tex2lyx is
updated to support format >= 257 (with InsetCaption).

	* src/Layout.cpp (forCaption): new method. Returns a pointer to a newly
	allocated Layout suitable for representing a caption.

	* src/tex2lyx/tex2lyx.cpp (tex2lyx): allocate a captionlayout object

	* src/tex2lyx/text.cpp: use the captionlayout object to recognize the
	\caption command.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@18962 a592a061-630c-0410-9148-cb99ea01b6c8