Rename "PanelEngine" to "MenuEngine"

Summary: Ref T11957.

Test Plan:
Grepped for "PanelEngine", renamed everything except "PanelEditEngine".

Grepped for these changed symbols:

```
ispanelengineconfigurable
getprofilepanelengine
setprofilepanelengine
setpanelengine
getpanelengine
PhabricatorProfilePanelEditEngine
```

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11957

Differential Revision: https://secure.phabricator.com/D17025

src/applications/project/controller/PhabricatorProjectBoardViewController.php

@@ -935,7 +935,7 @@ final class PhabricatorProjectBoardViewController
     $set_default = $request->getBool('default');
     if ($set_default) {
       $this
-        ->getProfilePanelEngine()
+        ->getProfileMenuEngine()
         ->adjustDefault(PhabricatorProject::PANEL_WORKBOARD);
     }