Refactor global member warnings.

--ignore-global now only toggles between showing global class or hiding it.

--warnings=-global turns warnings about global members on/off regardless of
the previous option.

Printing of the warnings is also no more duplicated in both lint.rb and app.rb.

Class#all_members and Class#all_local_members no more filter out private members.
Small refactoring inside Class to avoid confusing use of all_members as local
variable.