Convert menu structure into data

[bclothier]

This arose from a tangential comment in the Peek Definition PR.

Currently, we make use of enum to describe how to layout our menus. This is less than optimal for at least 3 reasons:

1. We're basically duplicating the names; as a member of an enum and as a menu command. This kind of violates the DRY principles and also thwarts any compile-time checks for the consistency of the menu structure.

2. The menu layout is technically more data than code.

3. We are mixing concerns --- we describe the layout in an enum but then we describe the BeginGroup as an override in the corresponding command.

This was developed with a earlier version of C#, and we need to work with commandbar system (blargh) so we can't use XAML or something similar.

However, with C# now featuring nameof() operator, we could consider converting the enum into a static class. Something like this:

public static class MenuLayout
{
   private static readonly List<string> _list => new List<string> 
    {
      nameof(OneCommand),
      nameof(AnotherCommand),
      default, //indicates a begin group?
      nameof(MoreCommand)
     }

   public static string GetElement(int index) => _list.Index(index);
}

The advantage gained is that the nameof operator affords us the type safety and compile-time validation and ability to refactor the command's name without breaking the menu layout.

Note: I suppose it could be a List<Command> instead of List<string> but I'm not 100% sure if there's a need to make it stringified or something. I also am not 100% sold on inserting default as a stand-in for inserting a separator (e.g. BeginGroup).

Would that help improvement our management of the menu layout?

Originally posted by @bclothier in #5751 (comment)

[mansellan]

Ah yes, I recall not liking this when I did the VB6 work.... Glad it's not just me!