Convert JSON document into a set of menus

[lyonsil]

As an owner/developer of an application that runs on Platform.Bible, I need to be able to define what menus to display in the UI for my application. This issue is one of several that will provide this capability. The other issues (not yet created) will focus on how the JSON document is created/provided to the platform. This issue is purely focused on converting that JSON document to UI.

This was created as part of #236.

The menu contribution design is still in progress, but the part of the design that doesn't seem to be changing or controversial is that the process should produce a final JSON document defining menus, and the UI should be able to render those menus on demand.

There are 2 types of menus defined in the spec:

1. Multi-column menus (like the main application menu) where each column has a header
2. Single-column menus without a header

There is a top-level, main application menu and a (main menu, context menu) pair per tab. The JSON schema is in the spec, but here's a copy, too. You don't need to programmatically use the schema for this issue. The schema is just here to help clarify the shape of the JSON document you should expect to receive.

menu-schema.txt

Write the code with the idea in mind that additional properties will be added to the menu item definitions over time. For example, we'll probably add shortcut key combinations in the future. You obviously can't/shouldn't try to write code that does something useful with undefined properties. Just don't blow up if extra properties are included.

[lyonsil]

Note: localization is out of scope

[lyonsil]

Pulled out of Sprint 15 because there is some lingering concern about the impact of "application" scope on the design.

[lyonsil]

Duplicate of #424