Add PrefService to gateway

Created prefs() convenience method for accessing the PrefsService.

Author: hinerm

src/main/java/org/scijava/AbstractGateway.java

@@ -53,6 +53,7 @@
 import org.scijava.platform.PlatformService;
 import org.scijava.plugin.AbstractRichPlugin;
 import org.scijava.plugin.PluginService;
+import org.scijava.prefs.PrefService;
 import org.scijava.script.ScriptService;
 import org.scijava.service.Service;
 import org.scijava.text.TextService;
@@ -188,6 +189,10 @@ public PluginService plugin() {
 		return get(PluginService.class);
 	}
 
+	public PrefService prefs() {
+		return get(PrefService.class);
+	}
+
 	@Override
 	public RecentFileService recentFile() {
 		return get(RecentFileService.class);