Improve support for overriding default menu items

[freakboy3742]

Derived from #2244.

Some features related to the default menu items:

• Commands can now be defined with an ID, and retrieved by that ID
• The generic system commands that Toga support (About, Visit Homepage, Exit and Preferences) have constants that are used as IDs
• A generic "wrap this function as a handler" method has been added, removing the need to explicitly re-define a menu command method that adds a single "command" argument that is then ignored.
• The App interface has a new preferences() method. If the user overrides this method, it will be used as the implementation of the Preferences menu item; if undefined, the menu item will still exist, but will be disabled. This isn't a full implementation of Settings API for Toga #90, but it does provide the ability for users to hook into the menu item.
• Handler methods now return their results. This isn't really useful at runtime, but it simplifies a lot of testing cases as you now have direct access to the underlying return results.

When combined with #2619, an app is able to delete or modify all the default menu items by a clear reference. The only exception to this are the ones on the Cocoa backend that are "window system related" (e.g., Hide, Close Window, Cut and Paste) which are included to provide basic window management, and shouldn't require any user override.

Refs #90.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

#2619 has caused a merge conflict on this file.

This line in particular seems inconsistent with #2619, which moved the create_app_commands out of the backends and into _startup.

[freakboy3742]

Yeah - this was the part I mentioned yesterday about PRs starting to step on each other.

[mhsmith]

OK, I've removed the redundant line.