Fix forward reference to enumeration types in GtkDoc header

It's invalid to forward-declare enumerations, yet they might be
referenced by typedefs.

Fix this by outputting enumerations first so typedefs can references
them.  As enumerations can't reference other types, it's safe to place
them before anything else.

Closes #952.
Closes #955.

scripts/gen-api-gtkdoc.py

@@ -362,6 +362,7 @@ def main(args):
     root = transform(doc)
 
     other = []
+    enums = []
     typedefs = []
 
     c_files = root.xpath(".//compounddef[@kind='file']/compoundname[substring(.,string-length(.)-1)='.c']/..")
@@ -376,7 +377,7 @@ def main(args):
 
         for n0 in f.xpath(".//*/memberdef[@kind='enum' and @prot='public']"):
             e = DoxyEnum.from_memberdef(n0)
-            other.append(e)
+            enums.append(e)
 
     for n0 in root.xpath(".//compounddef[@kind='struct' and @prot='public']"):
         e = DoxyStruct.from_compounddef(n0)
@@ -404,12 +405,21 @@ def main(args):
         outfile.write("typedef struct TMSourceFile TMSourceFile;\n")
         outfile.write("typedef struct TMWorkspace TMWorkspace;\n")
 
-        # write typedefs first, they are possibly undocumented but still required (even
+        # write enums first, so typedefs to them are valid (as forward enum declaration
+        # is invalid).  It's fine as an enum can't contain reference to other types.
+        for e in filter(lambda x: x.is_documented(), enums):
+            outfile.write("\n\n")
+            outfile.write(e.to_gtkdoc())
+            outfile.write(e.definition)
+            outfile.write("\n\n")
+
+        # write typedefs second, they are possibly undocumented but still required (even
         # if they are documented, they must be written out without gtkdoc)
         for e in typedefs:
             outfile.write(e.definition)
             outfile.write("\n\n")
 
+        # write the rest (structures, functions, ...)
         for e in filter(lambda x: x.is_documented(), other):
             outfile.write("\n\n")
             outfile.write(e.to_gtkdoc())