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