cookbook: Fix build of the examples

We don't have a tests/data directory any more since the test suites
reorganization; the cookbook examples, though, rely on the existence of
the redhand.png image. In order to fix them, we copy the file in the
examples directory, and we reference it directly. Since we need it for
the examples, and we install the example code, it's also necessary to
add the image to the EXTRA_DIST rule.

doc/cookbook/examples/events-mouse-scroll.c

@@ -68,7 +68,7 @@ main (int argc, char *argv[])
   ClutterActor *viewport;
   ClutterActor *texture;
 
-  gchar *image_file_path = TESTS_DATA_DIR "/redhand.png";
+  const gchar *image_file_path = "redhand.png";
 
   if (argc > 1)
     {