Fix description string extraction

Reusing needs_plural for extracting plural descriptions was a bad idea -
it means that all the item descriptions were erroneously marked as
needing plural forms.

Add a new category for types neeeding plural descriptions, and use this
instead.

lang/extract_json_strings.py

@@ -191,7 +191,6 @@ def warning_supressed(filename):
     "BOOK",
     "COMESTIBLE",
     "ENGINE",
-    "event_statistic",
     "GENERIC",
     "GUN",
     "GUNMOD",
@@ -204,6 +203,11 @@ def warning_supressed(filename):
     "WHEEL",
 }
 
+# These objects use a plural form in their description
+needs_plural_desc = {
+    "event_statistic"
+}
+
 # these objects can be automatically converted, but use format strings
 use_format_strings = {
     "technique",
@@ -1220,8 +1224,9 @@ def extract(item, infilename):
             c = "Description for {}".format(name)
         else:
             c = None
-        if object_type in needs_plural:
-            writestr(outfile, item["description"], comment=c, pl_fmt=True, **kwargs)
+        if object_type in needs_plural_desc:
+            writestr(outfile, item["description"], comment=c, pl_fmt=True,
+                    **kwargs)
         else:
             writestr(outfile, item["description"], comment=c, **kwargs)
         wrote = True