plug-ins: Don't translate "ms" for WEBP animation

Resolves #15632
Since our code expects WEBP time to be measured in
ASCII "ms", translating it causes the time between
frames to be ignored. This patch reuses the string from
the GIF loading plug-in so that we always add milliseconds
as "MS", per Daniel Plakhotich.

plug-ins/file-webp/file-webp-load.c

@@ -224,7 +224,9 @@ load_image (GFile                  *file,
               goto error;
             }
 
-          name = g_strdup_printf (_("Frame %d (%dms)"), frame_num, iter.duration);
+          name = g_strdup_printf (_("Frame %d (%d%s)"),
+                                  frame_num, iter.duration, "ms");
+
           create_layer (image, outdata, 0, name, width, height);
           g_free (name);