Improve menu \ command bar configuration

[mansellan]

Currently, we have a fragmented approach to defining menu and command bar layouts for:

• COM menu bars
• COM command bars
• WPF toolbars
• WPF context menus

COM items are registered for IoC, have metadata specified in code (*MenuItem classes), and are ordered using reflected enums. WPF toolbars and context menus are defined directly in XAML.

Let's abstract and unify this, such that a command hierarchy can be specified explicitly using a single approach, injected into any container, and observed for events.

This is a placeholder description, I will expand on the specifics.