Added decorators for adding callbacks to menus

[Mahi]

This allows one to define the menu before its callbacks while keeping the code clean.

[Ayuto]

This just moves a single line...

def my_callback(...):
    pass
menu.select_callback = my_callback

menu.register_select_callback
def my_callback(...)
    pass

I would rather update the menus package to support overrideable methods instead of callbacks.

[Mahi]

You're right and it's just a feature suggestion instead of an actual improvement. But I find it much more beautiful and clear than the current version (think of a very long function where menu.select_callback = my_callback will get lost in the clutter quite easily, unless you separate it with an empty line or two). And it comes at the cost of nothing! But you guys decide :)

I would rather update the menus package to support overrideable methods instead of callbacks.

Absolutely. But until someone has enough time and patience to do that...