GtkBuilder XML uses enum nicknames, full names, or integer values, but
we accept GIR names, so passing those through doesn't work if the name
has an underscore (which traditionally turns into a dash in the
nickname). Avoid the problem by always writing the integer value of the
enum member.
When only one flag is specified, it's parsed as an IdentValue rather
than a FlagsValue, so IdentValue needs to be able to type check flags
(aka bitfields) properly.
Fixes#47
