feat(wm): add theme socket message

This commit adds a new SocketMessage::Theme which allows for themes to
be set programmatically. This change has also been plumbed through to
komorebi-bar so that the bar theme will also update after komorebi
processes the message and passes it on to subscribers.

A new theme_manager module has been introduced to add notification-based
handling of theme changes, both from the static config file being
updated and from SocketMessage::Theme being received.