Adds new component `ChannelControlItem` and it's child components to be
used as the clickable control in the new chat sidebar for joined
channels.
Has public properties `HasUnread` and `MentionCount` to control the
display of the channel having unread messages or mentions of the user.
Channel select/join requests are exposed via `OnRequestSelect` and
`OnRequestLeave` events respectively which should be handled by a parent
component.
Requires a cached `Bindable<Channel>` instance to be managed by a parent
component.
Requires a cached `OveralayColourScheme` instance to be provided by a
parent component.
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