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/java/org/lucares/pdbui/PropertyKeys.java

@@ -11,4 +11,20 @@ public interface PropertyKeys {
 	 * Path for temporary files
 	 */
 	String TMP_DIR = "path.tmp";
+
+	/**
+	 * The number of seconds generated images shall be cached.
+	 */
+	String CACHE_IMAGES_DURATION_SECONDS = "cache.images.duration.seconds";
+
+	/**
+	 * Default value for {@link PropertyKeys#CACHE_IMAGES_DURATION_SECONDS}
+	 */
+	String CACHE_IMAGES_DURATION_SECONDS_DEFAULT = "3600";
+
+	/**
+	 * Indicates whether or not this instance is running in production. This
+	 * property is used to switch Vue.js into production or development mode.
+	 */
+	String PRODUCTION_MODE = "mode.production";
 }