Message/Command bus integration.

Search sub-components needed a way to share their state with the parent
search component. This is accomplished via eventing. Each component
still maintains the only copy of its state, but whenever that state
changes it notifies the search component via the events channel and
'filterUpdated'.

The search filters component also taps these events to decide which tags
to display.

Next I need to do the same eventing but for 'filterRemoved'.