Allow plugins to receive field classes

Costs cannot directly import, e.g., the SelectField, so we want the
service to be able to return the registered classes in order for Costs
to register the SelectField for a new type (costObject).

frontend/app/components/wp-edit/wp-edit-field/wp-edit-field.module.ts

@@ -69,7 +69,11 @@ export class FieldFactory {
     return new fieldClass(workPackage, fieldName, schema);
   }
 
-  protected static getType(type:string):string {
+  public static getClassFor(fieldName:string):typeof Field {
+    return this.classes[fieldName];
+  }
+
+  public static getType(type:string):string {
     let fields = FieldFactory.fields;
     let defaultType = FieldFactory.defaultType;
 

frontend/app/components/wp-edit/wp-edit-field/wp-edit-field.service.ts

@@ -41,12 +41,17 @@ export class WorkPackageEditFieldService {
     return FieldFactory.create(resource, fieldName, schema);
   }
 
-  public addFieldType(fieldClass, displayType:string, fields:string[]) {
+  public addFieldType(fieldClass:typeof Field, displayType:string, fields:string[]) {
     fieldClass.type = displayType;
     fieldClass.$injector = this.$injector;
     FieldFactory.register(fieldClass, fields);
     return this;
   }
+
+  public extendFieldType(displayType:string, fields:string[]) {
+    var fieldClass = FieldFactory.getClassFor(displayType);
+    return this.addFieldType(fieldClass, displayType, fields);
+  }
 }
 
 //TODO: Use 'wpEdit' module