Add support for keyboard navigation for submenu item

[peterblazejewicz]

This changes minor details in the BlazoredSubMenu implemenation to
support:

• keyboard based navigation (role and tabindex)
• keyboard based interaction (Enter key down)

As the support for event propagation in Blazor is not yet there, the
implementation marks support for SPACE keyboard as TODO item (supporting
SPACE from code requires cancelling propagation of the event in the
handler).

The changes do not impact samples details.

Thanks!

[chrissainty]

Thanks for doing this @peterblazejewicz. I don't know if you've seen, but .NET Core Preview 2 is out and Blazor now supports stopping event propagation. Would you like to make the changes you suggested before I merge this?

[peterblazejewicz]

Would you like to make the changes you suggested before I merge this?

Yes, let me review the docs. The best, if we could use the focused trap only for Space and Enter (I believe they discussed this as part of discussion on how to implement these new apis).
Will update and ping back,
Thanks!

[peterblazejewicz]

@chrissainty
So I've amended the PR, but limited changes to removing references to the event bubbling implementation details and changing to a simple solution, that seems to work for most use cases.
I think one should not use the current, opaque 'preventDefault' attributes, as this is not something desired here with the specific key handler. For example: I'd like to prevent default event when Spacebar has been pressed (as there is a scroll event associated with space bar being down) and catch the up to toggle the menu. I cannot just prevent all events, as this breaks supporting events like built-in browser shortcuts while there is a focus on the menu.
Let's revisit the details in the future.
Thanks!

[chrissainty]

This is great, thanks @peterblazejewicz.

I haven't had a chance to test out the new event management bits in Preview 2 yet. But it sounds like they are a bit all or nothing which might not be great.