Add hashes to images

Upon successfull reading an image file, this computes a SHA1 hash of the
image and saves it with the picture tag in the log file. When a file is
not successfully loaded (for example because the log was created on a
different computer) we look up the hash in a dictionary that maps hashes
to local file names.

That dictionary (actually two for both directions), is loaded on startup
and saved upon destruction of the main window.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

dive.h

@@ -366,6 +366,7 @@ struct dive_components {
 /* picture list and methods related to dive picture handling */
 struct picture {
 	char *filename;
+	char *hash;
 	offset_t offset;
 	degrees_t latitude;
 	degrees_t longitude;