This commit adds:
* An `AwesomeBarState` class to hold the currently visible suggestions
and last clicked suggestion.
* `VisibilityStateUpdated` and `SuggestionClicked` `AwesomeBarAction`s
to update the state, and an `EngagementFinished` action to reset the
state. Consumers will dispatch these actions on the `BrowserStore`
in response to user interactions with the awesomebar and toolbar.
* A `metadata` map to `AwesomeBar.Suggestion`, allowing providers to
attach opaque data to each suggestion.
c4350f91fe