Modularize global quick create builtin items

Summary: Ref T5867. Instead of hard-coding projects, tasks and repositories, let EditEngines say "I want a quick create item" so third-party code can also hook into the menu without upstream changes.

Test Plan: Saw same default items in menu.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5867

Differential Revision: https://secure.phabricator.com/D17215

src/applications/diffusion/editor/DiffusionRepositoryEditEngine.php

@@ -20,6 +20,10 @@ final class DiffusionRepositoryEditEngine
     return false;
   }
 
+  public function isDefaultQuickCreateEngine() {
+    return true;
+  }
+
   public function getEngineName() {
     return pht('Repositories');
   }