VUI: Keyboard input

[kpreid]

Right now, all keyboard events are interpreted by InputProcessor as character control. We need to have other modes, which are selected based on the current UI state:

• Keyboard navigation of menus/dialogs.

  Making this complete will require adding a notion of focused widget, which is its own complex problem, but we can add the mode without necessarily making it do anything yet.

• Text input

  In some cases (web at least), text input can and should be handled by focusing on a native text box, which gets us IME support, copy/paste, etc. for free, and means we don't need to redirect key events ourselves. But in the general platform-agnostic case we don't have that option.