enable client side caching for plot requests

Doesn't work perfectly yet, because the height/width sometimes changes
by one or two pixels.

pdb-ui/src/main/resources/application.properties

@@ -4,4 +4,6 @@ db.base=${base.dir}/db
 path.tmp=${base.dir}/tmp
 path.output=${base.dir}/out
 
+cache.images.duration.seconds=3600
+
 logging.config=classpath:log4j2.xml