Adds components `ChannelListing` and `ChannelListing` item with visual
test. Essentially a more simplified version of the existing
`ChannelSelectionOverlay` component.
Correctly implements `IFilterable` behaviour to filter child channel
items. Channel joined state is based on the underlying `Joined` bindable
of the `Channel` class.
Channel join/leave events are exposed via `OnRequestJoin` and
`OnRequestLeave` events which should be handled by parent component.
Requires a cached `OverlayColourScheme` instance to be provided by the
parent overlay component when added.
