(concatenate_label): new, concatenate two labels in an intelligent way
(convert): add the TocLevel for sectioning styles
(convert): convert LabelString and StringAppendix for sectioning
styles (append counter)
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@13533 a592a061-630c-0410-9148-cb99ea01b6c8