Add keyboard support in uikit

[ensisoft]

trafficstars

This is a large ticket and will likely need to be split into smaller ones.

• Create a concept of virtual keys so that the application can map some native keyboard events into uikit virtual keys that the UI can react on.
• Create a concept of "input focus", i.e. which widget has the input focus for receiving key events.
  • focus indication
• React on some uikit virtual keys and perform tasks such as "move focus to next widget", "move focus to prev widget".
  • focus order (also called tab order) of widgets