Add flags to the serialization protocol

It still needs no versioning, as it's not really used by anyone.

An alternative method of passing a "low-quality" flag would be
perusing fiv_thumbnail_key_lq from fiv-thumbnail.c, which would
create a circular dependency, unless fiv_io_{de,}serialize*()
were moved to fiv-thumbnail.c.

fiv-thumbnail.h

@@ -45,10 +45,10 @@ typedef struct _FivThumbnailSizeInfo {
 	const char *thumbnail_spec_name;    ///< thumbnail-spec directory name
 } FivThumbnailSizeInfo;
 
-extern FivThumbnailSizeInfo fiv_thumbnail_sizes[FIV_THUMBNAIL_SIZE_COUNT];
-
 enum { FIV_THUMBNAIL_WIDE_COEFFICIENT = 2 };
 
+extern FivThumbnailSizeInfo fiv_thumbnail_sizes[FIV_THUMBNAIL_SIZE_COUNT];
+
 /// If non-NULL, indicates a thumbnail of insufficient quality.
 extern cairo_user_data_key_t fiv_thumbnail_key_lq;