Since we don't always have direct reference to the WorkbenchMenuService,
and it makes no sense to provide a public interface for a bug (537046)
in the design, we define internal IMenuServiceWorkaround interface and
let SlaveMenuService and WorkbenchMenuService implement it.

Change-Id: I21a2a592b1305b955d2d1814f2845a84bda0a681
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>