Make it possible to configure the trigger in the pipeline code

Author: Guy Veraghtert

src/main/java/stashpullrequestbuilder/stashpullrequestbuilder/StashBuildTrigger.java

@@ -43,6 +43,7 @@
 import jenkins.model.ParameterizedJobMixIn.ParameterizedJob;
 import net.sf.json.JSONObject;
 import org.apache.commons.lang.StringUtils;
+import org.jenkinsci.Symbol;
 import org.kohsuke.stapler.AncestorInPath;
 import org.kohsuke.stapler.DataBoundConstructor;
 import org.kohsuke.stapler.DataBoundSetter;
@@ -76,9 +77,6 @@ public class StashBuildTrigger extends Trigger<Job<?, ?>> {
 
   private transient StashPullRequestsBuilder stashPullRequestsBuilder;
 
-  @Extension
-  public static final StashBuildTriggerDescriptor descriptor = new StashBuildTriggerDescriptor();
-
   @DataBoundConstructor
   public StashBuildTrigger(
       String projectPath,
@@ -342,6 +340,8 @@ public boolean isOnlyBuildOnComment() {
     return onlyBuildOnComment;
   }
 
+  @Extension
+  @Symbol("stashBuildTrigger")
   public static final class StashBuildTriggerDescriptor extends TriggerDescriptor {
     public StashBuildTriggerDescriptor() {
       load();