ui: Add Viewer API for plugins

- Add 'Viewer', an imperative interface for plugins to manipulate the
  UI.
- Following the pattern of engine and store each plugin gets a
  ViewerProxy - allowing us to blame API usage on a specific plugin
  and dispose() the proxy, stopping the plugin from using it.
- Initially add sidebar APIs for show, hide, isVisible
- Migrate toggle sidebar command into a plugin using the Viewer API.
- Migrate existing methods of toggling the sidebar to use the command.
- Also make EngineProxy disposable.

Change-Id: I1d668af7da64b6d5e6d29cc493562f5fdf7df404