Add Keyboard shortcut support to Vuu UI

[heswell]

Vuu UI should make it possible for application authors to define keyboard shortcuts for an application. This will entail several pieces of work

• Build a UI that display available shortcuts to user.
• Create a registration mechanism, where an action can be associated with a shortcut
• Detect keyboard events at application level, determine whether a matching shortcut had been registered and, if so, invoke the action associated with shortcut. Propagation of this event should be prevented.
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