Unexport AddMenuItem(); remove unused vars; fix warning.

AddMenuItem() was primarily used internally. It does not work well
after Initialize(), and its API is not friendly (id parameter is
misleading/poorly supported). The ability to modify menu items after
Initialize() will be brought back in a correct way later.
Fix "dock.m:10:4: warning: ibaction attribute can only be applied to
Objective-C instance methods [-Wignored-attributes]" warning.