Fix flag values
Enums/bitfields have a C identifier, a GLib nick, and a GIR name. The last two are usually the same except the nick uses '-' and the GIR name uses '_'. It is confusing.
Blueprint tries to use GIR names for everything, but GTK requires a C identifier or GLib nick for enum and bitfield values. Blueprint was sometimes passing the GIR name through instead of finding one of the other names. Fixed by always emitting the C identifier for enums and bitfields where the GIR type is known.