Add TEI annotation layer

[kuczera]

Can we add a new annotation layer for layout in TEI:
tei/layout/emph rend=”red” [red text]
tei/layout/expan abbr="" [blue text]
tei/layout/corr sic="original" [green underline]
tei/layout/del cause=“moved“, rend=”striked out”, unit="line" quantity="2" [striked out]
tei/layout/add place="margin, above etc." [orange text]
tei/layout/gap reason=“not readable“ [underlined white space]
tei/layout/surplus [brown underline]