Add "c:identifier" attribute to GIrNodeValue (for flags and enum values).

Add "c:identifier" attribute to GIrNodeValue (for flags and enum values).

Flags and enums with a GType have a value_nick and value_name
strings available in the class struct. But for flags and enums
without GType, we need to get this information from introspection.

g_base_info_get_name() gives the string for value_nick. In the GIR,
the attribute "c:identifier" is the string neede for value_name.

This patch adds the "c:identifier" from GIR to the typelib for all
flags and enum values. It can be retireved using
g_base_info_get_attribute(info, "c:identifier").

https://bugzilla.gnome.org/show_bug.cgi?id=642757