Allow using icon names in InsetInfo

Allow using icon names in InsetInfo

Currently, only lfun names are accepeted but there are cases in which
the lfun name does not map directly to an icon. Fore example, in the
outliner, the icon named "promote" is used for the lfun outline-out.
As a result, a graphics inset is used in the documentation for describing
the corresponding icon. Now one can also use an icon name. The argument
of "info-isert icon" is firstly lookep up as an lfun. If no corresponding
icon is found, the argument is taken as the name of the icon. If no such
icon exists, the "unknown" icon is used.