Dinamically adding and removing commands and menu

[daniele77]

It's useful adding and removing commands and submenus during the working session. For example, when loading/unloading a plugin at run-time we should be able to add/remove the related cli commands.

To implement this, maybe we should get rid of std::unique_ptr for commands and menus. We need to return an handle when adding commands/submenus, to remove them at a later time.