Each plugin contributes a list single of menus, which are expected
to be merged in existing top menu entries if they already exist:
as stated in the doc and already implemented in GWT:
"Plugins can also add additional menu items to Gerrit’s top menu
entries by defining a MenuEntry that has the same name as a Gerrit
top menu entry"
Change-Id: I8540b93b71e9c9c1f043e16d2076bade4de9a49a