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