Add option 'Ingore' to Maven -> Import -> Generated sources folder

plugins/maven/src/main/java/org/jetbrains/idea/maven/importing/MavenFoldersImporter.java

@@ -237,6 +237,9 @@ public class MavenFoldersImporter {
 
         addAllSubDirsAsSources(targetDir, isTestSources);
         break;
+
+      case IGNORE:
+        break; // Ignore.
     }
   }
 

plugins/maven/src/main/java/org/jetbrains/idea/maven/project/MavenImportingSettings.java

@@ -53,6 +53,7 @@ public class MavenImportingSettings implements Cloneable {
   private List<Listener> myListeners = ContainerUtil.createLockFreeCopyOnWriteList();
 
   public enum GeneratedSourcesFolder {
+    IGNORE("Don't detect"),
     AUTODETECT("Detect automatically"),
     GENERATED_SOURCE_FOLDER("target/generated-sources"),
     SUBFOLDER("subdirectories of \"target/generated-sources\"");