Handle enumerations with the full range of signed and unsigned values

Handle enumerations with the full range of signed and unsigned values

The C compiler will pick an enumeration type that accomodates the specified
values for the enumeration, so ignoring 64-bit enumerations, we can
have enumeration values from MININT32 to MAXUINT32. To handle this properly:

 - Use gint64 for holding eumeration values when scanning
 - Add a 'unsigned_value' bit to ValueBlob so we can distinguish the
   int32 vs. uint32 cases in the typelib
 - Change the return value of g_value_info_get_value() to gint64.

https://bugzilla.gnome.org/show_bug.cgi?id=629704