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.

tests/samples/flags.blp

@@ -6,5 +6,5 @@ Gio.Application {
 }
 
 EventControllerScroll {
-  flags: vertical;
+  flags: both_axes;
 }