File format change.
This allows for the relevant InsetCommand-based dialogs (such as
citation text before/after, Bibitem label, hyperlink name etc.)
to provide both the input of verbatim code or text that is transformed
to proper LaTeX code.
Some dialogs (Nomencl, Href) already had some methods (although they
could not be toggled), which are now centralized and streamlined.
The initial work of this patch has been done by Georg Baum (see
http://www.lyx.org/trac/attachment/ticket/2751/x.diff)
Fixes: #2751, #8227.
