xml: Fix flags and enums

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.

blueprintcompiler/language/values.py

@@ -168,6 +168,20 @@ class NumberValue(Value):
 class Flag(AstNode):
     grammar = UseIdent("value")
 
+    @property
+    def name(self) -> str:
+        return self.tokens["value"]
+
+    @property
+    def value(self) -> T.Optional[int]:
+        type = self.parent.parent.value_type
+        if not isinstance(type, Enumeration):
+            return None
+        elif member := type.members.get(self.tokens["value"]):
+            return member.value
+        else:
+            return None
+
     @docs()
     def docs(self):
         type = self.parent.parent.value_type